fixing a math error in the documentation about flex-grid
fixing a math error in the documentation about flex-grid

Jazz-Man/bourbon,nickg33/bourbon,Nikeshsuwal/bourbon,greyhwndz/bourbon,san650/bourbon,ismarkunc/bourbon,jamadam/bourbon-docs-jp,jamadam/bourbon-docs-jp,cllns/bourbon,Jazz-Man/bourbon,jamadam/bourbon-docs-jp,jamadam/bourbon-docs-jp,nagyistoce/sass-bourbon.io,chriseppstein/bourbon,smithdamen/bourbon,chriseppstein/bourbon,nickg33/bourbon,jasonwalkow/bourbon,thoughtbot/bourbon,mindeavor/bourbon,mindeavor/bourbon,craftsmen/bourbon,zyj1022/bourbon,zyj1022/bourbon,jamesmenera/bourbon-libsass,craftsmen/bourbon,setie/bourbon,greyhwndz/bourbon,jamesmenera/bourbon-libsass,jasonwalkow/bourbon,setie/bourbon,thoughtbot/bourbon,smithdamen/bourbon