Omit the trailing space in self-closing XML tags

[jeffhandley]

Per https://github.com/dotnet/runtime/pull/48841/files#r585232027, I recommend we change the XML tag generation to omit the trailing space in self-closing tags. That would align with the behavior that VS provides when authoring XML doc comments, even though it's different from what we've done in dotnet-api-docs.

[gewarren]

Same as #59?

[carlossanlop]

Duplicate of #59