move sphinx h3 to '-' instead of '^'

In the sphinx document h3 is supposed to be '-' not '^':

=, for sections
-, for subsections
^, for subsubsections

We have to enforce consistency here because we're processing included
files which all have to agree, otherwise it's a sphinx error.

Part of bp:api-ref-in-rst

Change-Id: Ic6eef5cacb07870f161b04b031e332f2b87aeedc

api-ref/source/os-assisted-volume-snapshots.inc

@@ -23,7 +23,7 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
@@ -41,7 +41,7 @@ Request
    :language: javascript
 
 Response
-^^^^^^^^
+--------
 
 .. rest_parameters:: parameters.yaml
 
@@ -70,7 +70,7 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
@@ -85,5 +85,5 @@ Query Parameters
   - delete_info: delete_info
 
 Response
-^^^^^^^^
+--------