docs: Fixes typo in library name flask-restx

clnrest installation instruction on the markdown tries to install `flask_restx` not `flask-restx`.

Reference: clnrest plugin complains of flask_restx missing (ElementsProject#7383)

Changelog-None.

doc/getting-started/getting-started/installation.md

@@ -596,7 +596,7 @@ Installation steps for clnrest are:
 
 ```
 sudo apt-get install python3-json5 python3-flask python3-gunicorn
-pip3 install --user flask-cors flask_restx pyln-client flask-socketio gevent gevent-websocket
+pip3 install --user flask-cors flask-restx pyln-client flask-socketio gevent gevent-websocket
 ```
 
 ### wss-proxy