Revise document. string to boolean.

doc/api/pipelines.md

@@ -14,7 +14,7 @@ GET /projects/:id/pipelines
 | `scope`   | string  | no       | The scope of pipelines, one of: `running`, `pending`, `finished`, `branches`, `tags`; |
 | `status`  | string  | no       | The status of pipelines, one of: `running`, `pending`, `success`, `failed`, `canceled`, `skipped`; |
 | `ref`     | string  | no       | The ref of pipelines |
-| `yaml_errors`| string  | no       | If true, returns only yaml error pipelines |
+| `yaml_errors`| boolean  | no       | Returns pipelines which have an error of gitlab-ci.yml |
 | `name`| string  | no       | The name of user who triggered pipelines |
 | `username`| string  | no       | The username of user who triggered pipelines |
 | `order_by`| string  | no       | Return requests ordered by `id`, `status`, `ref`, or `user_id` fields. Default is `id` |