Correct comment in zlib.h on os setting in gzip header.

The comment said that the os is set to 255, when in fact it has
been set to the current os since zlib 1.2.3. Or at least our best
guess at the os made at compile time.

zlib.h

@@ -821,8 +821,9 @@ ZEXTERN int ZEXPORT deflateSetHeader(z_streamp strm,
    gzip file" and give up.
 
      If deflateSetHeader is not used, the default gzip header has text false,
-   the time set to zero, and os set to 255, with no extra, name, or comment
-   fields.  The gzip header is returned to the default state by deflateReset().
+   the time set to zero, and os set to the current operating system, with no
+   extra, name, or comment fields.  The gzip header is returned to the default
+   state by deflateReset().
 
      deflateSetHeader returns Z_OK if success, or Z_STREAM_ERROR if the source
    stream state was inconsistent.