[rest api] fix purchases documentation (#380)

Documentation in the REST OpenAPI spec were showing incorrect information about Purchasing endpoints.

Update the open api spec to reflect information about purchases, not requests.

openapi.yaml

@@ -171,7 +171,7 @@ components:
           type: string
           description: Random data
 
-    StorageRequestState:
+    Purchase:
       type: object
       properties:
         state:
@@ -317,29 +317,29 @@ paths:
 
   "/storage/purchases/{id}":
     get:
-      summary: "Returns information about node's Requests"
+      summary: "Returns purchase details"
       tags: [ Marketplace ]
-      operationId: getStorageRequestState
+      operationId: getPurchase
       parameters:
         - in: path
           name: id
           required: true
           schema:
             type: string
-          description: Hexadecimal ID of a Request
+          description: Hexadecimal ID of a Purchase
       responses:
         "200":
-          description: Information about the Request state and specification
+          description: Purchase details
           content:
             application/json:
               schema:
                 type: array
                 items:
-                  $ref: "#/components/schemas/StorageRequestState"
+                  $ref: "#/components/schemas/Purchase"
         "400":
-          description: Invalid or missing Request ID
+          description: Invalid or missing Purchase ID
         "404":
-          description: Request ID not found
+          description: Purchase not found
         "503":
           description: Purchasing is unavailable