include_src parameter in example; Whitespace cleanup

In the apt::source example, include the include_src parameter.  Clean-up
whitespace in the example to be consistent with the style guide.

README.md

@@ -51,6 +51,7 @@ apt::source { "debian_unstable":
   required_packages => "debian-keyring debian-archive-keyring",
   key               => "55BE302B",
   key_server        => "subkeys.pgp.net",
-	pin => "-10"
+  pin               => "-10",
+  include_src       => true
 }
 </pre>