Rewording bullet point about custom use of functional colours

Author: seaemsi

source/changes-to-govuk-frontend-v6/index.html.md.erb

@@ -42,7 +42,7 @@ You might need to update some items to update to v6.0.0, including:
 
 You'll need to check some items still work as expected, including:
 
-- making sure any custom colours or custom components that reference our functional colours still have good contrast and legibility
+- making sure any custom use of functional colours still follows best accessibility practice for contrast and legibility
 - updating any custom header or footer content, as we've changed how you can use and customise these components
 - testing the new type scale in your service to see if you need to make any adjustments