Corrections for comments in plugins/windowmenu/windowmenu.c

As it looks like non-developers are not allowed to directly issue commits or merge requests (which is fine), I'd like to propose this way the following corrections to comments found in plugins/windowmenu/windowmenu.c:

• change "an utf-8 valid name" into "a utf-8 valid name" (appears three times, plus the same issue exists in plugins/pager/pager-buttons.c, where it also should be corrected)

• One comments says "leave when we same xfw screen was picked". I suppose it is meant to say "leave when the same xfw screen was picked". Please correct the comment as appropriate.

Thanks