Fix a typo in md.c

Hi,

I've attached the patch for $subject. In the following comment,

        /*
        * If available and useful, use posix_fallocate() (via FileAllocate())
         * to extend the relation. That's often more efficient than using
         * write(), as it commonly won't cause the kernel to allocate page
         * cache space for the extended pages.
         *
        * However, we don't use FileAllocate() for small extensions, as it
         * defeats delayed allocation on some filesystems. Not clear where
         * that decision should be made though? For now just use a cutoff of
         * 8, anything between 4 and 8 worked OK in some local testing.

s/FileAllocate()/FileFallocate()/

Regards,

-- 
Masahiko Sawada
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