github/issue_template: unify spellings

"behavior" is written in US English form in all templates except here.

.github/ISSUE_TEMPLATE/6_question.md

@@ -11,7 +11,7 @@ Don't ask questions about issues, errors or problems you have and instead open
 a proper issue with the right template from the previous selection.
 
 This template is meant for questions about the workings of mpv, to clarify about
-unspecified behaviour of options or command, or anything else that is not well
+unspecified behavior of options or command, or anything else that is not well
 described and needs clarification.
 
 Before asking a question make sure it hasn't been asked or answered yet.