fix typo

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

Author: sjaeckel

doc/crypt.tex

@@ -1546,7 +1546,7 @@ \chapter{Stream Ciphers}
 err = rc4_stream_done(&st);
 \end{verbatim}
 
-To encrypt plaintext (or decrypt ciphertext) using RC6 for data already in
+To encrypt plaintext (or decrypt ciphertext) using RC4 for data already in
 memory with a single function call, the following function may be used.
 \begin{verbatim}
 err = rc4_stream_memory(key, keylen, datain, datalen, dataout);