Ensure that .pm6 is used consistently in module-packages.pod6.

Author: cfa

doc/Language/module-packages.pod6

@@ -112,8 +112,8 @@ say Foo::loud-greeting;  # OUTPUT: «GREETINGS, CAMELIA!␤»
 say friendly-greeting;   # OUTPUT: «Greetings, friend!␤»
 =end code
 
-If C<Foo.pm> is placed deeper within the source tree, e.g. at
-C<lib/Utils/Foo.pm>, we can elect to name the module C<Utils::Foo>
+If C<Foo.pm6> is placed deeper within the source tree, e.g. at
+C<lib/Utils/Foo.pm6>, we can elect to name the module C<Utils::Foo>
 to maintain consistency.