Update docs to mention HTTPHeaderMap.

Author: Lukasa

docs/source/advanced.rst

@@ -143,7 +143,7 @@ the original response, or when also processing the original response in a
 separate thread (N.B. do not do this; ``hyper`` is not yet thread-safe)::
 
     conn.request('GET', '/')
-    response = conn.getheaders()
+    response = conn.getresponse()
     for push in conn.getpushes(): # all pushes promised before response headers
         print(push.path)
     conn.read()

docs/source/quickstart.rst

@@ -72,13 +72,12 @@ HTTP/2 *stream identifier*. If you're planning to use ``hyper`` in this very
 simple way, you can choose to ignore it, but it's potentially useful. We'll
 come back to it.
 
-Once you've got the data, things continue to behave exactly like
-``http.client``::
+Once you've got the data, things diverge a little bit::
 
     >>> resp.getheader('content-encoding')
     'deflate'
-    >>> resp.getheaders()
-    [('x-xss-protection', '1; mode=block')...
+    >>> resp.headers
+    HTTPHeaderMap([(b'x-xss-protection', b'1; mode=block')...
     >>> resp.status
     200