Update modules.rakudoc

Link url has a typo (should be `/language/filename-extensions`, not  `/Language/...`

Author: finanalyst

doc/Language/modules.rakudoc

@@ -51,7 +51,7 @@ L<synopsis S11|https://design.raku.org/S11.html#Units> says: Confusing? Yes it
 is.> , a C<t> directory for tests, and possibly a C<bin> directory for
 executable programs and scripts.
 
-See L<Language/filename-extensions> for current and historic
+See L<filename extensions|/language/filename-extensions> for current and historic
 extensions for various file types.
 
 =head1 X<Loading and basic importing|Language,compunit>