In the documentation of osrm-prepare [1] and in the general documentation for running osrm [2], it is written that osrm-prepare takes 2 filenames as arguments, but with the last version of OSRM, it fails:
$ ./osrm-prepare cantal-rectangle.osrm cantal-rectangle.osrm.restrictions [warn] Only one file can be specified

In a previous version, osrm-prepare was already using one single file, the *.osrm one, and the restrictions files was found.

So, it should be a small error in the doc ;-)

[1] https://github.com/DennisOSRM/Project-OSRM/wiki/osrm-prepare.1
[2] https://github.com/DennisOSRM/Project-OSRM/wiki/Running-OSRM