doc/muxers: correct description of hls_enc_key and hls_enc_iv

because hls_enc_key and hls_enc_iv get 16byte char
for example:
-hls_enc_key 0123456789abcdef -hls_enc_iv abcdefghijklmnop

Reviewed-by: Gyan Doshi <ffmpeg@gyani.pro>
Signed-off-by: Steven Liu <liuqi05@kuaishou.com>

doc/muxers.texi

@@ -831,7 +831,7 @@ When enabled every segment generated is encrypted and the encryption key
 is saved as @var{playlist name}.key.
 
 @item -hls_enc_key @var{key}
-Hex-coded 16byte key to encrypt the segments, by default it
+16-octet key to encrypt the segments, by default it
 is randomly generated.
 
 @item -hls_enc_key_url @var{keyurl}
@@ -839,7 +839,7 @@ If set, @var{keyurl} is prepended instead of @var{baseurl} to the key filename
 in the playlist.
 
 @item -hls_enc_iv @var{iv}
-Hex-coded 16byte initialization vector for every segment instead
+16-octet initialization vector for every segment instead
 of the autogenerated ones.
 
 @item hls_segment_type @var{flags}