Fixing doctor messages which expect bash string interpolation to replace $PATH in .bash_profile

The one-liner suggested by doctor to fix the issue when homebrew's ```/bin``` appears after ```/usr/bin``` doesn't work because ```$PATH``` never gets substituted in the string. Hence changing the single quoted to double quotes to allow bash to interpolate ```PATH```'s value.

Running the one-liner the way it currently is, causes the PATH to be reset to ```usr/local/bin``` like:
```bash
$ echo $PATH
/usr/local/bin
```
when it actually should've been like this:
```bash
$ echo $PATH
/usr/local/bin:/usr/bin:..
```