Update readme about overriding exec

```
let g:syntastic_css_csslint_exe='~/.vim/node_modules/.bin/csslint' 
```
not working, but
```
g:syntastic_css_csslint_exec='~/.vim/node_modules/.bin/csslint'
```
works fine.
And according to this function https://github.com/scrooloose/syntastic/blob/master/plugin/syntastic/checker.vim#L51
there must be exec instead of exe