Update AFURLRequestSerialization.h (#4613)

Modify comments.

AFNetworking/AFURLRequestSerialization.h

@@ -278,7 +278,7 @@ forHTTPHeaderField:(NSString *)field;
 #pragma mark -
 
 /**
- The `AFMultipartFormData` protocol defines the methods supported by the parameter in the block argument of `AFHTTPRequestSerializer -multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:`.
+ The `AFMultipartFormData` protocol defines the methods supported by the parameter in the block argument of `AFHTTPRequestSerializer -multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:error:`.
  */
 @protocol AFMultipartFormData