Fix use of reserved characters in wp-cli.yml

[kellymears]

I don't think the generated wp-cli.yml is compliant with the yml spec.

The “@” (x40, at) and “`” (x60, grave accent) are reserved for future use.

Reserved indicators can't start a plain scalar.

— yaml spec (1.2.2)

yamllint and vscode's default YAML checks report errors, so I think I might be on to something.

test.yml
  1:1       error    syntax error: found character '@' that cannot start any token (syntax)

[swalkinshaw]

alias_test.go needs to be updated but otherwise this looks good 👍

[chrillep]

@swalkinshaw when will this be released? 😅

[swalkinshaw]

@chrillep I'll do one this week (maybe tonight).