* doc/syntax/calling_methods.rdoc: fix old operator for safe navigation
	  operator. [ci skip][fix GH-1182] Patch by @dougo


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@54393 b2dd03c8-39d4-4d8f-98ff-823fe69b080e