Edit target doc template to remove email

We don't really want to communicate with target maintainers via email.
GitHub is where everything happens, people should have a GitHub account
that can be pinged on issues.

This doesn't necessarily have to be a strict rule, but edit the template
to suggest this. The previous template made it look like we care about
having an email address, which we do not.

src/doc/rustc/src/platform-support/TEMPLATE.md

@@ -6,7 +6,7 @@ One-sentence description of the target (e.g. CPU, OS)
 
 ## Target maintainers
 
-- Some Person, `[email protected]`, https://github.com/...
+- Some Person, https://github.com/...
 
 ## Requirements