Fix append_to_zshrc

This PR fixes a bug recently introduced by the new append_to_zshrc function in [this commit](https://github.com/thoughtbot/laptop/commit/4e7f598feea1668140395caea325bcc1adbc7470) by @kenyonj. 

"\n" were introduced and not escaped when this function was called. I then used echo -e instead of printf to properly interpret the input string.