Fixing find hooks script call error in docs/hooks.txt

- maintenance/findhooks.php -> maintenance/findHooks.php
- ./maintenance/... -> php maintenance/... (*)

* Our maintenance files aren't released as executable, so this kind of
instruction could be confusing for UNIX system administration novices.

Change-Id: I5e1e76c46b3a8a4c7168c0b5715ede648044bd44