pow: fix installation caveats.

Add the backslashes in the caveats of Pow formula. The backslashes are
necessary otherwise if the user copies the commands and pastes them in
Terminal, the space of "Application Support" breaks the command.

Reverse the order of ln command in order to respect the documentation of
Pow which says that the .pow directory in user home must be a symbolic link
of Hosts directory. See the note of section 2.1
(http://pow.cx/manual.html#section_2.1).

Closes #23366.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>