Fix Documentation build due to rename of main.c to mtrr.c

This fixes this documentation build error that is due to a file rename:

  Error: Cannot open file ../arch/x86/kernel/cpu/mtrr/main.c

Fixes: 0afe832e ("Merge branch 'x86-cleanups-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip"

)
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>