Update sendmail_path in README.md

[JosephGabito]

Newer version of Go is adding the mhsendmail in ~/go/bin/mhsendmail.

The sendmail_path should be (path)/go/bin/mhsendmail

to get the (path), cd to ~/go/bin and type pwd, copy the output and append mhsendmail to it.

For example:

(myuser is an example user, you'll get different user in your end.)

1. cd ~/go/bin, this will output: /Users/myuser/go/bin
2. Open php.ini, find sendmail_path and change the value to /Users/myuser/go/bin/mhsendmail.

Thanks