Update web_api.md

the request type is POST in file_manager.py
```python
self.server.register_endpoint(
            "/server/files/metascan", RequestType.POST, self._handle_metascan_request
        )
```

docs/web_api.md

@@ -2529,7 +2529,7 @@ been scanned the endpoint will force a rescan
 
 HTTP request:
 ```http
-GET /server/files/metascan?filename={filename}
+POST /server/files/metascan?filename={filename}
 ```
 
 JSON-RPC request: