fix: single pin I/O in POST operations

This removes arrays and replaces them with single object in places
which were missed before when we made the switch to single ops.
Context:
#39 (comment)

Closes #46

ipfs-pinning-service.yaml

@@ -181,7 +181,7 @@ paths:
           content:
             application/json:
               schema:
-                $ref: '#/components/schemas/PinResults'
+                $ref: '#/components/schemas/PinStatus'
         '400':
           $ref: '#/components/responses/BadRequest'
         '401':
@@ -228,9 +228,7 @@ paths:
         content:
           application/json:
             schema:
-              type: array
-              items:
-                $ref: '#/components/schemas/Pin'
+              $ref: '#/components/schemas/Pin'
       responses:
         '202':
           description: Accepted