Fix the caveats which blocks the good installation of Pow.

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).

Before: 
```bash
mkdir -p ~/Library/Application Support/Pow/Hosts
ln -s ~/.pow ~/Library/Application Support/Pow/Hosts
```
After: 
```bash
mkdir -p ~/Library/Application\ Support/Pow/Hosts
ln -s ~/Library/Application\ Support/Pow/Hosts ~/.pow
```