Fix minor typo in zlib.h comments [Rześniowiecki].

zlib.h

@@ -1578,9 +1578,8 @@ ZEXTERN uLong ZEXPORT crc32   OF((uLong crc, const Bytef *buf, uInt len));
 /*
      Update a running CRC-32 with the bytes buf[0..len-1] and return the
    updated CRC-32.  If buf is Z_NULL, this function returns the required
-   initial value for the for the crc.  Pre- and post-conditioning (one's
-   complement) is performed within this function so it shouldn't be done by the
-   application.
+   initial value for the crc.  Pre- and post-conditioning (one's complement) is
+   performed within this function so it shouldn't be done by the application.
 
    Usage example: