rpm: Fix version number in .spec file

This code should be build as version 2.5.3, not 2.5.2 (as in
debian/changelog, CMakeLists.txt or changelog in the mfs.spec file), so
this commit fixes the version number in the .spec file.

Change-Id: Ia8c4fc79d958e6f6f9b90c9b1391a4e85aea10ef