Fix typos in README (#455)
Fix typos in README (#455)


Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt