Changed import vue-echo to vue-tawk on README file

[ttiagojm]

On the README file, this line import Tawk from 'vue-echo' when executed give an error because we doesn't install vue-echo, we installed vue-tawk. So I changed this line to import Tawk from 'vue-tawk'.

And I add a example on the TAWK_SRC , because people you may think it's the code snippet that Tawk gives us and it's not , it's the src(link) inside the code snippet.