doc/metadata: fix error in timebase description

Fixes #7184.

doc/metadata.texi

@@ -33,7 +33,7 @@ At the beginning of a chapter section there may be an optional timebase to be
 used for start/end values. It must be in form
 @samp{TIMEBASE=@var{num}/@var{den}}, where @var{num} and @var{den} are
 integers. If the timebase is missing then start/end times are assumed to
-be in milliseconds.
+be in nanoseconds.
 
 Next a chapter section must contain chapter start and end times in form
 @samp{START=@var{num}}, @samp{END=@var{num}}, where @var{num} is a positive