Fix typo on var names in grid styl docs

As stated on the docs of styl:
http://learnboost.github.com/stylus/docs/variables.html
Vars go with `=`

If the docs were followed then the example did not work. It would appear as
a fixed-width grid
Merge pull request #9 from joakin/grid_typos_in_var_names

Fix typo on var names in grid styl docs