Changes sub to routine

Actually, methods and subroutines are usually listed under
"methods", however 'routine' and not 'sub' is normally used
there. This is not peculiar to this file, there are `sub`s elsewhere
in the documentation; however, they will need their very own issue,
because this closes #683

Author: JJ

doc/Type/Str.pod6

@@ -478,7 +478,7 @@ Examples:
     "Perl".flip;  # RESULT: «lreP»
     "ABBA".flip;  # RESULT: «ABBA»
 
-=head2 sub sprintf
+=head2 routine sprintf
 
  multi sub sprintf( Str:D $format, *@args --> Str:D)