Fix duplicated 'for' in MemoryExtensions.xml (dotnet#5368)

Author: jo-ninja

xml/System/MemoryExtensions.xml

@@ -25,7 +25,7 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>Provides extension methods for for the memory- and span-related types, such as <see cref="T:System.Memory`1" />, <see cref="T:System.ReadOnlyMemory`1" />, <see cref="T:System.Span`1" />, and <see cref="T:System.ReadOnlySpan`1" />.</summary>
+    <summary>Provides extension methods for the memory- and span-related types, such as <see cref="T:System.Memory`1" />, <see cref="T:System.ReadOnlyMemory`1" />, <see cref="T:System.Span`1" />, and <see cref="T:System.ReadOnlySpan`1" />.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>