doc: fix syntax for durations and timestamps.

Omitting the seconds has not worked for a long time, if ever.
Omitting the minutes too is just nonsensical for a duration
(it is indistinguishable from just seconds).

doc/ffmpeg.texi

@@ -164,7 +164,7 @@ streams are delayed by @var{offset} seconds.
 Set the recording timestamp in the container.
 The syntax for @var{time} is:
 @example
-now|([(YYYY-MM-DD|YYYYMMDD)[T|t| ]]((HH[:MM[:SS[.m...]]])|(HH[MM[SS[.m...]]]))[Z|z])
+now|([(YYYY-MM-DD|YYYYMMDD)[T|t| ]]((HH:MM:SS[.m...])|(HHMMSS[.m...]))[Z|z])
 @end example
 If the value is "now" it takes the current time.
 Time is local time unless 'Z' or 'z' is appended, in which case it is

doc/filters.texi

@@ -3254,7 +3254,7 @@ number or a valid video frame rate abbreviation. The default value is
 @item duration, d
 Set the video duration of the sourced video. The accepted syntax is:
 @example
-[-]HH[:MM[:SS[.m...]]]
+[-]HH:MM:SS[.m...]
 [-]S+[.m...]
 @end example
 See also the function @code{av_parse_time()}.