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>

Documentation/core-api/kernel-api.rst

@@ -284,7 +284,7 @@ Resources Management
 MTRR Handling
 -------------
 
-.. kernel-doc:: arch/x86/kernel/cpu/mtrr/main.c
+.. kernel-doc:: arch/x86/kernel/cpu/mtrr/mtrr.c
    :export:
 
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