Update en-GB.com_templates.ini

To make the error message more helpful/user friendly when user tries to delete a template in the Template Manager rather than the Extension Manager.

To test: Try to delete a non-default template using the delete button in the Template Manager. You should now receive instructions how it should be done!
If I'm not mistaken, we usually write ` -> ` instead of '> ` (e.g. `Extensions -> Extension manager ->`). Or, is it `Extensions => Extension manager =>`?