doc/muxers.texi: properly insert {}s in example

use '@{' / '@}', fixes compile errors:
*** '{' without macro. Before: 1:-'.'}
*** '}' without opening '{' before:

since:
907ac20a avformat/hlsenc: added HLS encryption
Signed-off-by: James Zern <jzern@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/muxers.texi

@@ -310,7 +310,7 @@ http://server/file.key
 Example shell script:
 @example
 #!/bin/sh
-BASE_URL=${1:-'.'}
+BASE_URL=$@{1:-'.'@}
 openssl rand 16 > file.key
 echo $BASE_URL/file.key > file.keyinfo
 echo file.key >> file.keyinfo