selftests/x86/fsgsbase: Fix a comment in the ptrace_write_gsbase test

A comment was unclear.  Fix it.

Fixes: 5e7ec857 ("selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/901034a91a40169ec84f1f699ea86704dff762e4.1593192140.git.luto@kernel.org