Fix DateTime.Subtract summary (#3970)

Author: gewarren

xml/System/DateTime.xml

@@ -5933,7 +5933,7 @@ The following example demonstrates the <xref:System.DateTime.Parse%28System.Stri
         <AssemblyVersion>4.2.0.0</AssemblyVersion>
       </AssemblyInfo>
       <Docs>
-        <summary>Returns a new <see cref="T:System.DateTime" /> that subtracts the specified time or duration from the value of this instance.</summary>
+        <summary>Returns the value that results from subtracting the specified time or duration from the value of this instance.</summary>
       </Docs>
     </MemberGroup>
     <Member MemberName="Subtract">
@@ -5979,7 +5979,7 @@ The following example demonstrates the <xref:System.DateTime.Parse%28System.Stri
       </Parameters>
       <Docs>
         <param name="value">The date and time value to subtract.</param>
-        <summary>Returns a new <see cref="T:System.DateTime" /> that subtracts the specified date and time from the value of this instance.</summary>
+        <summary>Returns a new <see cref="T:System.TimeSpan" /> that subtracts the specified date and time from the value of this instance.</summary>
         <returns>A time interval that is equal to the date and time represented by this instance minus the date and time represented by <paramref name="value" />.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[