Update ri manpage to show how to access ruby pages (#1340)

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Author: adam12

man/ri.1

@@ -36,6 +36,8 @@ can be:
 .It Class::method | Class#method | Class.method | method
 .Pp
 .It gem_name: | gem_name:README | gem_name:History
+.Pp
+.It ruby: | ruby:NEWS | ruby:globals
 .El
 .Pp
 All class names may be abbreviated to their minimum unambiguous form.