Fix bad XML causing Except function doc comments to not work

[sbeca]

Some incorrect XML was causing doc comments for the Except functions to not show up at all in IDEs and causing the following to output into doc XML files of downstream projects:

<!-- Badly formed XML comment ignored for member "M:PolyfillExtensions.Except``1(System.Collections.Generic.IEnumerable{``0},``0)" -->
<!-- Badly formed XML comment ignored for member "M:PolyfillExtensions.Except``1(System.Collections.Generic.IEnumerable{``0},``0[])" -->
<!-- Badly formed XML comment ignored for member "M:PolyfillExtensions.Except``1(System.Collections.Generic.IEnumerable{``0},``0,System.Collections.Generic.IEqualityComparer{``0})" -->
<!-- Badly formed XML comment ignored for member "M:PolyfillExtensions.Except``1(System.Collections.Generic.IEnumerable{``0},System.Collections.Generic.IEqualityComparer{``0},``0[])" -->

This PR fixes the XML so that it works correctly.

[SimonCropp]

thanks