declare $string so xt test passes

Author: tbrowder

doc/Type/Str.pod6

@@ -907,6 +907,7 @@ one does this:
     
 Note that new characters can be inserted as well:
 
+    my $string = 'azc';
     $string.substr-rw(2, 0) = "-Zorro-"; # insert new characters BEFORE the character at index 2
     $string.say;                         # OUTPUT: «az-Zorro-c␤»