extensions help: make disabling example analogous to enabling example

The example for enabling an extension in the module search path is:

  foo =

foo can only be disabled by:

  foo = !

and not by:

  hgext.foo = !

As the examples now omit "hgext." the example

  hgext.bar = !

is misleading.

help/extensions.txt

@@ -28,6 +28,6 b' scope, prepend its path with !::'
 
   [extensions]
   # disabling extension bar residing in /path/to/extension/bar.py
-  hgext.bar = !/path/to/extension/bar.py
+  bar = !/path/to/extension/bar.py
   # ditto, but no path was supplied for extension baz
-  hgext.baz = !
+  baz = !