skel/.shrc: Improve commented CDPATH example for POSIX requirements.
skel/.shrc: Improve commented CDPATH example for POSIX requirements.

POSIX says an empty entry in CDPATH shall not result in the new directory
being printed, while any non-empty entry shall result in the new directory
being printed, including ".". Therefore, the value of CDPATH should almost
always start with a colon, not dot and colon.

Our sh does not print the name for empty entries as well as "." entries.

MFC after:	1 week

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