Link to SSLContext.sni_callback, instead of set_servername_callback

SSLContext.sni_callback is a better replacement for
set_servername_callback (that was added in part because of my whining
about trio bug python-triogh-11). Let's link to it instead.

This also fixes a silly issue where we were using :meth: to link to
SSLContext.set_servername_callback, but in the 3.7 docs that's
documented as being an attribute. Even though it's a method. Oh well,
whatever, if we don't link to it we don't have to care.

trio/_ssl.py

@@ -328,8 +328,8 @@ class SSLStream(Stream):
     attributes are re-exported as methods and attributes on this class.
 
     This also means that if you register a SNI callback using
-    :meth:`~ssl.SSLContext.set_servername_callback`, then the first argument
-    your callback receives will be a :class:`ssl.SSLObject`.
+    :obj:`~ssl.SSLContext.sni_callback`, then the first argument your callback
+    receives will be a :class:`ssl.SSLObject`.
 
     """