lib/mutex_m.rb: fix example code in documentation

* lib/mutex_m.rb: [DOC] fix invalid example code to make it
  syntax highlighted.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mutex_m.rb

@@ -25,12 +25,12 @@
 #   obj.extend Mutex_m
 #
 # Or mixin Mutex_m into your module to your class inherit Mutex instance
-# methods ---remember to call super() in your class initialize method.
+# methods --- remember to call super() in your class initialize method.
 #
 #   class Foo
 #     include Mutex_m
-#     def initialize ...
-#       ...
+#     def initialize
+#       # ...
 #       super()
 #     end
 #     # ...