contrib/gregbook: fix syntax errors

[jwilk]

[tangyaofang]

Obvious grammatical mistakes, thank you for discovering.

[jbowler]

The first change is correct but hardly matters; this is documentation, not code. The second change appears to be wrong and misleading; the point of the code is to protect against overflow, the code correctness crowd can complain about this way of doing it but the original seems correct to me.