doc: Fix names of av_log color environment variables.

doc/avtools-common-opts.texi

@@ -119,8 +119,8 @@ Set the logging level used by the library.
 By default the program logs to stderr, if coloring is supported by the
 terminal, colors are used to mark errors and warnings. Log coloring
 can be disabled setting the environment variable
-@env{FFMPEG_FORCE_NOCOLOR} or @env{NO_COLOR}, or can be forced setting
-the environment variable @env{FFMPEG_FORCE_COLOR}.
+@env{AV_LOG_FORCE_NOCOLOR} or @env{NO_COLOR}, or can be forced setting
+the environment variable @env{AV_LOG_FORCE_COLOR}.
 The use of the environment variable @env{NO_COLOR} is deprecated and
 will be dropped in a following Libav version.