doc/codecs: add missing comma to introduction

Signed-off-by: Marth64 <marth64@proxyid.net>

doc/codecs.texi

@@ -3,7 +3,7 @@
 @c man begin CODEC OPTIONS
 
 libavcodec provides some generic global options, which can be set on
-all the encoders and decoders. In addition each codec may support
+all the encoders and decoders. In addition, each codec may support
 so-called private options, which are specific for a given codec.
 
 Sometimes, a global option may only affect a specific kind of codec,