docs: fix dir headline level (#2665)

The other commands in this section are on headline level 4, which probably is also the expected one for `dir`.

Author: JensErat

website/src/docs/reference/schema.md

@@ -637,7 +637,7 @@ tasks:
       - go build -ldflags="-s -w" ./...
 ```
 
-### `dir`
+#### `dir`
 
 - **Type**: `string`
 - **Description**: The directory in which this task should run