[BUGFIX] Change private network static ip in Vagrant file

Change static ip from 192.168.23.4 to 192.168.23.6 as mentioned in the Readme.rst file.

192.168.23.4 is used in the TYPO3 Neos Vagrant box [1]
192.168.23.3 is used in the TYPO3 Flow Vagrant box [2]

I don't know if 192.168.23.5 is already used for something else. So i stick with the ip mentioned in the Readme.rst file: 192.168.23.6

[1] https://github.com/tlayh/vagrant-typo3neos
[2] https://github.com/tlayh/vagrant-typo3flow