Fix title of '~' as string concatenation.

Author: fluca1978

doc/Language/5to6-nutshell.pod6

@@ -708,7 +708,7 @@ my $result = ( $score > 60 ) ?  'Pass' :  'Fail'; # Perl 5
 my $result = ( $score > 60 ) ?? 'Pass' !! 'Fail'; # Perl 6
 =end code
 
-=head2 C<.> (Dot op) Concatenation
+=head2 C<.> (Dot) String Concatenation
 
 Replaced by the tilde.