Remove additional backticks in upgrading document
Remove additional backticks in upgrading document

It was already inside of a code block, implying that it would be a shell call 
from php, while it is just a function call.
michabbb-backup/guzzle,michabbb-backup/guzzle,kkopachev/guzzle,guzzle/guzzle,kkopachev/guzzle,guzzle/guzzle