Add missing :nodoc: comment

We were missing a `:nodoc:` magic comment that was making automated tools show
that this method was missing documentation, when it really didn't need to be
documented.
by Devon Estes <devon.c.estes@gmail.com>
https://github.com/ruby/ruby/pull/1482 fix GH-1482

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57047 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/pp.rb

@@ -417,8 +417,8 @@ class Range # :nodoc:
   end
 end
 
-class String
-  def pretty_print(q)
+class String # :nodoc:
+  def pretty_print(q) # :nodoc:
     lines = self.lines
     if lines.size > 1
       q.group(0, '', '') do