Fix prooftree formula

The prooftree configure file does not like having an equals after prefix but requires a space instead.

Fixes the error:
```
==> Downloading http://askra.de/software/prooftree/releases/prooftree-0.10.tar.gz
Already downloaded: /Library/Caches/Homebrew/prooftree-0.10.tar.gz
==> ./configure --prefix=/usr/local/Cellar/prooftree/0.10

Recognized options are:
  --prefix <path>	installation prefix [/usr/local]
  --bindir <path>	user executables [PREFIX/bin]
  --mandir <path>	man pages [PREFIX/share/man]
```