managen: fix the option sort order

... it used to strip off the .d file extension to sort correctly but
ever since the extension changed to .md the operation failed and the
sort got wrong.

Follow-up to 2494b8d

Closes #13567

scripts/managen

@@ -984,7 +984,7 @@ sub listglobals {
 
 sub noext {
     my $in = $_[0];
-    $in =~ s/\.d//;
+    $in =~ s/\.md//;
     return $in;
 }