Reword "\n" -> vertical newline.

Author: fluca1978

doc/Language/variables.pod6

@@ -1083,7 +1083,7 @@ X<|$?GRAMMAR>X<|$?TABSTOP>X<|$?NL>X<|$?ENC>
     $?ROLE      Which role am I in? (as variable)
     $?GRAMMAR   Which grammar am I in?
     $?TABSTOP   How many spaces is a tab in a heredoc or virtual margin?
-    $?NL        What "\n" means: LF, CR or CRLF
+    $?NL        What a vertical newline "\n" means: LF, CR or CRLF
     $?ENC       Default encoding of Str.encode/Buf.decode/various IO methods.
 
 With particular regard to the C<$?NL>, see the  L<newline pragma|/language/pragmas>.