Remind users to include the @ symbol when applying license file

*Original comment by @ppf2:*

Seems to be a common mistake out there.  Some admins tend to leave out the @ symbol before the path for the curl command that applies the license.  Perhaps we can put an Important note or highlight the importance of not leaving out the @ before the filepath in the docs?

https://www.elastic.co/guide/en/x-pack/current/installing-license.html