_config.yml: no 'mailto' in 'email'

Closes carpentries/styles#283

`site.email` is used as:

_includes/lesson_footer.html:	        <a href="mailto:{{ site.email }}">Contact</a>
_includes/workshop_footer.html:	<a href="mailto:{{ site.email }}">Contact</a>

Therefore, it does not make sense to have `mailto` in the `email` itself.

Author: mr-c

bin/boilerplate/_config.yml

@@ -8,10 +8,8 @@ carpentry: "swc"
 # Overall title for pages.
 title: "Lesson Title"
 
-# Contact.  This *must* include the protocol: if it's an email
-# address, it must look like "mailto:[email protected]",
-# or if it's a URL, "https://gitter.im/username/ProjectName".
-email: "mailto:[email protected]"
+# Email address, no mailto:
+email: "[email protected]"
 
 #------------------------------------------------------------
 # Generic settings (should not need to change).