README: Update docs for Patchwork 3.0

Refer to the API version in Patchwork 3.0, not the rather old 2.1.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

README.rst

@@ -74,14 +74,14 @@ settings are **required**:
   The URL for the Patchwork instance's API. This should include the API
   version::
 
-      https://patchwork.ozlabs.org/api/1.1
+      https://patchwork.ozlabs.org/api/1.2
 
   You can discover the API version supported by your instance by comparing the
   server version, found at ``/about``, with the API versions provided in the
   `documentation`__. For example, if your server is running Patchwork version
-  2.1.x, you should use API version 1.1.
+  3.0.x, you should use API version 1.2.
 
-  __ https://patchwork.readthedocs.io/en/stable-2.1/api/rest/#rest-api-versions
+  __ https://patchwork.readthedocs.io/en/stable-3.0/api/rest/#rest-api-versions
 
 ``pw.project``
   The project name or list-id. This will appear in the URL when using the web