Use a pointer to state in _update and _final

The functions `crypto_hash_sha256_update` and `crypto_hash_sha256_final` take a
pointer to `state` as their first arguments, however the documentation showed
an example of passing state directly by-value.  This is incorrect and won't
compile.  This patch corrects the documentation by passing a pointer.
Merge pull request #4 from rmoore/fix-sha2-doc

Use a pointer to state in _update and _final