Improve wording about substr-rw aliasing

Thanks to itz++ for pointing out the sentence's meaning.

S32-setting-library/Str.pod

@@ -446,7 +446,8 @@ string variable:
  $string ~~ /(barney)/;
  substr-rw($string, $0.from, $0.to) = "fred";
 
-This writable reference can be aliased too, for repeated operations:
+This writable reference can be the target of an alias, for repeated
+operations:
 
  my $r := substr-rw($string, $0.from, $0.to);
  $r = "fred";   # "barney" replaced by "fred"