Escape ERB to fix the code block example

We need to esape the ERB command to have it rendered as a command in the
code block example. Not escaping the ERB means the command is being
executed, which is not the point of this content.

source/amend_project/content/index.html.md.erb

@@ -139,5 +139,5 @@ This warning text format is consistent with the [GOV.UK Design System warning te
 Insert the following Ruby code into your content file:
 
 ```ruby
-<%= warning_text('INSERT WARNING TEXT HERE') %>
+<%%= warning_text('INSERT WARNING TEXT HERE') %>
 ```