snprintf() and vsnprintf() return value comment

[vgrudenic]

Hi, just a tiny remark: the comment for snprintf/vsnprintf seems to be incorrect:

(returns) The number of characters that are WRITTEN into the buffer, not counting the terminating null character. If the formatted string is truncated the buffer size (count) is returned

The description in the readme file is ok:

Anyway, if the output was truncated due to this limit, the return value is the number of characters that could have been written. Notice that a value equal or larger than count indicates a truncation.

I made a quick update so I'll place a PR (I didn't find the -1 being returned from anywhere in case of errors, so I didn't mention it explicitly).

[mpaland]

Thanks a lot for the PR, Vedran.