Replace types string with str.

Uses command:

    ag -l 'type ([^:]+): string' | \
        xargs gsed -r -i.bak -e 's/type ([^:]+): string/type \1: str/g'

Note: [-r for gsed (GNU sed) is needed for group
matching](http://superuser.com/a/336819/125262).

Author: tswast

packages/google-cloud-bigtable/google/cloud/bigtable/client.py

@@ -317,7 +317,7 @@ def instance(self, instance_id, location=_EXISTING_INSTANCE_LOCATION_ID,
         :type instance_id: str
         :param instance_id: The ID of the instance.
 
-        :type location: string
+        :type location: str
         :param location: location name, in form
                          ``projects/<project>/locations/<location>``; used to
                          set up the instance's cluster.