Fix query parameter name comment in issue API (#4421)

public/swagger.v1.json

           {
             "type": "string",
             "description": "if provided, only comments updated since the provided time are returned.",
-            "name": "string",
+            "name": "since",
             "in": "query"
           }
         ],
           {
             "type": "string",
             "description": "if provided, only comments updated since the specified time are returned.",
-            "name": "string",
+            "name": "since",
             "in": "query"
           }
         ],

routers/api/v1/repo/issue_comment.go

 	//   description: index of the issue
 	//   type: integer
 	//   required: true
-	// - name: string
+	// - name: since
 	//   in: query
 	//   description: if provided, only comments updated since the specified time are returned.
 	//   type: string
 	//   description: name of the repo
 	//   type: string
 	//   required: true
-	// - name: string
+	// - name: since
 	//   in: query
 	//   description: if provided, only comments updated since the provided time are returned.
 	//   type: string