(doc) : fix salt.minion_config to point to correct file in OS X walkthrough

[This] (https://github.com/saltstack/salt/blob/1026dbea61c2cf58015ddc9ba9911f5fb03f4a9c/doc/topics/tutorials/walkthrough_macosx.rst#creating-the-minion-configuration-file) section of the OS X walkthrough instructs the developer to create the minion config file at ```/etc/salt/minion```. [Here] (https://github.com/saltstack/salt/blob/1026dbea61c2cf58015ddc9ba9911f5fb03f4a9c/doc/topics/tutorials/walkthrough_macosx.rst#modify-vagrantfile-to-use-salt-provisioner) it asks to developer to point ```salt.minion_config``` to ```./minion.conf```. This is inconsistent and gives the error ```The specified minion_config file could not be found. ``` when provisioning. Fix the example to point to the already created ```/etc/salt/minion``` file