Fix a mistake in registration_shared_secret_path docs (#15078)

* fix a typo in registration_shared_secret_path docs
Signed-off-by: Filip Rutar <filip.rutar@gmail.com>

* changelog

changelog.d/15078.doc

@@ -0,0 +1 @@
+Fix a mistake in registration_shared_secret_path docs.

docs/usage/configuration/config_documentation.md

@@ -2232,7 +2232,7 @@ key on startup and store it in this file.
 
 Example configuration:
 ```yaml
-registration_shared_secret_file: /path/to/secrets/file
+registration_shared_secret_path: /path/to/secrets/file
 ```
 
 _Added in Synapse 1.67.0._