Fix issue where error message would not print directive name properly

[mattvague]

Right now if I try to define a custom directive with a non-existent directive in .before() e.g. `Alpine.directive('foo', foo).before('baz') I get the following error:

alpinejs:5 Cannot find directive ${directive}. ${name} will use the default order of execution

Instead of

alpinejs:5 Cannot find directive baz. foo will use the default order of execution

The issue is that the error message is defined using double quotes instead of backticks. This PR fixes the issue and should now print the error correctly.