doc: Add example for Symbol#to_s

* string.c: add example for Symbol#to_s.

The docs for Symbol#to_s only include an example for
Symbol#id2name, but not for #to_s which is an alias;
the docs should include examples for both methods.

From: Marcus Stollsteimer <sto.mar@web.de>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57536 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -9707,6 +9707,7 @@ sym_inspect(VALUE sym)
  *  Returns the name or string corresponding to <i>sym</i>.
  *
  *     :fred.id2name   #=> "fred"
+ *     :ginger.to_s    #=> "ginger"
  */