fix Generated_Code entry in CSharp.gitignore

The existing entry for Generated_Code (with a space and then a comment afterwards) wasn't working for me (at least with msysgit version 1.7.11.msysgit.1).

Once I put the comment on a separate line and removed the trailing space after 'Generated_Code', it worked fine.

[The gitignore(5) man page](http://www.kernel.org/pub/software/scm/git/docs/gitignore.html) says a line starting with # is treated as a comment, but doesn't mention what # does if it's elsewhere in the line, so based on the previous behavior and trying a line of "Generated_Code#foo", it appears that the # and the rest of the line isn't ignored, so we need to move the comment to its own line that starts with #