Fix api ref link in TransfersClient

Api ref links in TransfersClient are broken, this is
to fix them.

No other broken links in current code.

Change-Id: I19be9d4afcb82b546cda093655d35c930f85c932
diff --git a/tempest/lib/services/volume/v2/transfers_client.py b/tempest/lib/services/volume/v2/transfers_client.py
index 6f21944..853948e 100644
--- a/tempest/lib/services/volume/v2/transfers_client.py
+++ b/tempest/lib/services/volume/v2/transfers_client.py
@@ -28,7 +28,7 @@
 
         For a full list of available parameters, please refer to the official
         API reference:
-        http://developer.openstack.org/api-ref/block-storage/v2/#create-volume-transfer-v2
+        https://developer.openstack.org/api-ref/block-storage/v2/#create-volume-transfer
         """
         post_body = json.dumps({'transfer': kwargs})
         resp, body = self.post('os-volume-transfer', post_body)
@@ -49,7 +49,7 @@
 
         For a full list of available parameters, please refer to the official
         API reference:
-        http://developer.openstack.org/api-ref/block-storage/v2/#list-volume-transfers-v2
+        https://developer.openstack.org/api-ref/block-storage/v2/#list-volume-transfers
         """
         url = 'os-volume-transfer'
         if params:
@@ -70,7 +70,7 @@
 
         For a full list of available parameters, please refer to the official
         API reference:
-        http://developer.openstack.org/api-ref/block-storage/v2/#accept-volume-transfer-v2
+        https://developer.openstack.org/api-ref/block-storage/v2/#accept-volume-transfer
         """
         url = 'os-volume-transfer/%s/accept' % transfer_id
         post_body = json.dumps({'accept': kwargs})