Use interpolation in examples

Author: andyw8

lib/rubocop/cop/rails/helper_instance_variable.rb

@@ -16,12 +16,12 @@ module Rails
       # @example
       #   # bad
       #   def welcome_message
-      #     "Hello " + @user.name
+      #     "Hello #{@user.name}"
       #   end
       #
       #   # good
       #   def welcome_message(user)
-      #     "Hello " + user.name
+      #     "Hello #{user.name}"
       #   end
       class HelperInstanceVariable < Cop
         MSG = 'Do not use instance variables in helpers.'.freeze

manual/cops_rails.md

@@ -825,12 +825,12 @@ example to a model, decorator or presenter.
 ```ruby
 # bad
 def welcome_message
-  "Hello " + @user.name
+  "Hello #{@user.name}"
 end
 
 # good
 def welcome_message(user)
-  "Hello " + user.name
+  "Hello #{user.name}"
 end
 ```