fix markdown syntax error

Update guides/source/plugins.md
Co-authored-by: NJonathan Hefner <jonathan@hefner.pro>

guides/source/plugins.md

@@ -390,7 +390,7 @@ class ActsAsYaffleTest < ActiveSupport::TestCase
 end
 ```
 
-Run the test to make sure the last two tests fail with an error that contains "NoMethodError: undefined method `squawk'",
+Run the test to make sure the last two tests fail with an error that contains "NoMethodError: undefined method \`squawk'",
 then update `acts_as_yaffle.rb` to look like this:
 
 ```ruby