Highlight only variable name in offense description

Author: andyw8

lib/rubocop/cop/rails/helper_instance_variable.rb

@@ -31,7 +31,7 @@ def on_ivar(node)
         end
 
         def on_ivasgn(node)
-          add_offense(node)
+          add_offense(node, location: :name)
         end
       end
     end

spec/rubocop/cop/rails/helper_instance_variable_spec.rb

@@ -16,7 +16,7 @@ def welcome_message
     expect_offense(<<-RUBY.strip_indent)
       def welcome_message(user)
         @user_name = user.name
-        ^^^^^^^^^^^^^^^^^^^^^^ Do not use instance variables in helpers.
+        ^^^^^^^^^^ Do not use instance variables in helpers.
       end
     RUBY
   end