doc: use POSIX shell syntax in the man page examples.

The shell examples given in the man page use the "haeredes <<< foo"
syntax to feed the string "foo" into haeredes on stdin. That's a
Bash-only shortcut for "echo foo | haeredes", however, and the latter
works in all POSIX-compatible shells. So, this commit updates the
documentation to use the more reliable syntax.