docs(toh-6): fix typos about Promise and Observable with Dart artifacts (angular#3436)

Author: osenvosem

public/docs/ts/latest/tutorial/toh-pt6.jade

@@ -366,8 +366,9 @@ h1 Providing HTTP Services
   But requests aren't always done only once.
   You may start one request,
   cancel it, and make a different request before the server has responded to the first request.
-  A *request-cancel-new-request* sequence is difficult to implement with *Promises*, but
-  easy with *Observables*.
+
+  A *request-cancel-new-request* sequence is difficult to implement with `Promise`s, but
+  easy with `Observable`s.
 
   ### Add the ability to search by name
   You're going to add a *hero search* feature to the Tour of Heroes.