Remind users to include the @ symbol when applying license file

[elasticmachine]

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