Update xxhash_spec.md

[davesnx]

Fix typo in xxhash_spec

[Cyan4973]

I don't think it's a typo.

Since, in the previous loop, the code consumed input 8 bytes at a time,
at the end of this loop, only [0-7] bytes remain.
Consequently the >=4 may happen once at a maximum.

[davesnx]

Totally, silly me.