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>
merge revision(s) 57536: [Backport #13192]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@61362 b2dd03c8-39d4-4d8f-98ff-823fe69b080e