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-security-group-default-rules.inc

@@ -19,14 +19,14 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
   - tenant_id: tenant_id
 
 Response
-^^^^^^^^
+--------
 
 .. rest_parameters:: parameters.yaml
 
@@ -57,7 +57,7 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
@@ -74,7 +74,7 @@ Request
    :language: javascript
 
 Response
-^^^^^^^^
+--------
 
 .. rest_parameters:: parameters.yaml
 
@@ -103,7 +103,7 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
@@ -111,7 +111,7 @@ Request
   - security_group_default_rule_id: security_group_default_rule_id
 
 Response
-^^^^^^^^
+--------
 
 .. rest_parameters:: parameters.yaml
 
@@ -140,7 +140,7 @@ Error response codes: computeFault(400, 500), serviceUnavailable(503), badReques
 unauthorized(401), forbidden(403), badMethod(405), itemNotFound(404)
 
 Request
-^^^^^^^
+-------
 
 .. rest_parameters:: parameters.yaml
 
@@ -148,5 +148,5 @@ Request
   - security_group_default_rule_id: security_group_default_rule_id
 
 Response
-^^^^^^^^
+--------