Update "9.3.1 The HEAD"

The old description in "9.3.1 The HEAD" confused HEAD with heads.
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         alternative modification
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When you run `git commit`, Git will first find the working branch HEAD points to, and then specify the parent of the new commit with the SAH-1 value stored in that working branch. Finally, the SAH-1 value stored in that working branch would be updated to the new commit object, while the HEAD file itself needs not to be changed.