[php] Fix php.ini COPY Documentation
Apparently COPY will expect a full target path nowadays:
```
Step 0 : FROM php:5.6-apache
---> 86db229ab259
Step 1 : COPY config/php.ini /usr/local/etc/php
stat /var/lib/docker/devicemapper/mnt/cb571a2599018afc008c08a509d201d4937eed74a22621efa94e3ef32bebc869/rootfs/usr/local/etc/php/php.ini: not a directory
```