docs: add paramter mutation_timeout to instance.table docs (#305)

Author: kolea2

google/cloud/bigtable/instance.py

@@ -643,6 +643,9 @@ def table(self, table_id, mutation_timeout=None, app_profile_id=None):
         :type table_id: str
         :param table_id: The ID of the table.
 
+        :type mutation_timeout: int
+        :param mutation_timeout: (Optional) The overriding mutation timeout.
+
         :type app_profile_id: str
         :param app_profile_id: (Optional) The unique name of the AppProfile.