Correct parameter names for (ReadOnly)Span.Slice(Int32, Int32) methods (#2468)

* Fix non-existent parameter `index` to `start`

* Fix non-existent, incorrect param names

Author: Gnbrkm41

xml/System/ReadOnlySpan`1.xml

@@ -712,7 +712,7 @@ A `ReadOnlySpan<T>` instance is often used to reference the elements of an array
       <Docs>
         <param name="start">The index at which to begin the slice.</param>
         <summary>Forms a slice out of the current read-only span that begins at a specified index.</summary>
-        <returns>A span that consists of all elements of the current span from <paramref name="index" /> to the end of the span.</returns>
+        <returns>A span that consists of all elements of the current span from <paramref name="start" /> to the end of the span.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="start" /> is greater than the number of items in the read-only span.</exception>
@@ -780,7 +780,7 @@ A `ReadOnlySpan<T>` instance is often used to reference the elements of an array
         <param name="start">The index at which to begin this slice.</param>
         <param name="length">The desired length for the slice.</param>
         <summary>Forms a slice out of the current read-only span starting at a specified index for a specified length.</summary>
-        <returns>A read-only span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="index" />.</returns>
+        <returns>A read-only span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="start" />.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="start" /> is less than zero or greater than <see cref="T:System.Span`1.Length" />.</exception>
@@ -888,4 +888,4 @@ If the `destination` length is shorter than `source`, no items are copied and th
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>

xml/System/Span`1.xml

@@ -919,7 +919,7 @@ Two <xref:System.Span%601> objects are equal if they have different lengths or i
       <Docs>
         <param name="start">The index at which to begin the slice.</param>
         <summary>Forms a slice out of the current span that begins at a specified index.</summary>
-        <returns>A span that consists of all elements of the current span from <paramref name="index" /> to the end of the span.</returns>
+        <returns>A span that consists of all elements of the current span from <paramref name="start" /> to the end of the span.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="start" /> is less than zero or greater than <see cref="T:System.Span`1.Length" />.</exception>
@@ -987,10 +987,10 @@ Two <xref:System.Span%601> objects are equal if they have different lengths or i
         <param name="start">The index at which to begin this slice.</param>
         <param name="length">The desired length for the slice.</param>
         <summary>Forms a slice out of the current span starting at a specified index for a specified length.</summary>
-        <returns>A span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="index" />.</returns>
+        <returns>A span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="start" />.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
-          <paramref name="start" /> or <paramref name="start" /> + <paramref name="index" /> is less than zero or greater than <see cref="T:System.Span`1.Length" />.</exception>
+          <paramref name="start" /> or <paramref name="start" /> + <paramref name="length" /> is less than zero or greater than <see cref="T:System.Span`1.Length" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToArray">
@@ -1106,4 +1106,4 @@ If `destination` is shorter than the source <xref:System.Span%601>, this method
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>