radeontop: remove outdated note from description

It Just Works™ without root these days.

pkgs/os-specific/linux/radeontop/default.nix

···
       blocks. Supports R600 and later cards: even Southern Islands should work.
       Works with both the open drivers and AMD Catalyst. Total GPU utilization
       is also valid for OpenCL loads; the other blocks are only useful for GL
-      loads. Requires root rights or other permissions to read /dev/mem.
+      loads.
     '';
     homepage = "https://github.com/clbr/radeontop";
     platforms = platforms.linux;