[Notifier] Fix documentation symfony notifier

[matthieudelmas]

Hi there 👋

After following the notifier doc to send slack notifications, I think there is a problem at the message customization step :

• I successfully get a slack notification, but when we go to the "Notifications" tab of the Symfony profiler I have an error message "Impossible to invoke a method ("getSubject") on a variable null"
  
• when i dump the created ChatMessage, i see that the notification is empty.
  

=> Reading the ChatMessage class, it seems that the static method fromNotification can solves this problem (check PR proposal)

The other fixes are more obvious:

• use RecipientInterface instead of SmsRecipientInterface since in the asChatMessage we use the RecipientInterface
• strval on price in the message since price is an integer
• method emoji on notification not on ChatMessage as the doc is telling us

(FYI this is my first public PR on github)

[javiereguiluz]

Thanks Matthieu ... and congrats on your first Symfony Docs contribution 🎉