locale.1: Use LC_TELEPHONE instead of LC_MESSAGES in the example

yesstr/nostr in LC_MESSAGES are planned to be changed at some
point [1], so it's better to provide an example which won't
be obsoleted by that change.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=16975

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>