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!
I can only find one example in the core language files
JFIELD_ALIAS_DESC="The Alias will be used in the SEF URL. Leave this blank and Joomla will fill in a default value from the title. This value will depend on the SEO settings (Global Configuration->Site).