change MUMPS' primary extension from .m to .mumps

.m currently clashes with Objective-C, and MUMPS is chosen before
Objective-C when creating .m files in Gist. Even if .mumps isn't a
legitimate MUMPS extension, I think we should change it anyway since
Objective-C is far more likely to be intended when a user uses .m

lib/linguist/languages.yml

@@ -787,7 +787,7 @@ M:
   lexer: Common Lisp
   aliases:
   - mumps
-  primary_extension: .m
+  primary_extension: .mumps
 Makefile:
   aliases: