Fix aws.plugins.zsh check rule

Check aws_zsh_completer.sh had exist or not

When aws_zsh_completer.sh is not exist, this will got error like this
`~/.oh-my-zsh/plugins/aws/aws.plugin.zsh:source:27: no such file or directory: aws_zsh_completer.sh`

I check it path, let it can work all the time.