docs(framework) Update copybutton behaviour in Flower documentation

[chongshenng]

Copying this code block

$ python server.py

Pasting before update: $ python server.py

Pasting after update: python server.py

---

Copying this code block

# Flower 1.8
$ flower-simulation \
    --server-app=sim:server_app \
    --client-app=sim:client_app \
    --num-supernodes=100

Pasting before update:

# Flower 1.8
$ flower-simulation \
    --server-app=sim:server_app \
    --client-app=sim:client_app \
    --num-supernodes=100

Pasting after update:

flower-simulation \
    --server-app=sim:server_app \
    --client-app=sim:client_app \
    --num-supernodes=100