Fix typo in BUILD.md [skip-ci]
Fix typo in BUILD.md [skip-ci]

ponylang/ponyc,ponylang/ponyc,sgebbie/ponyc,sgebbie/ponyc,kulibali/ponyc,jemc/ponyc,ponylang/ponyc,kulibali/ponyc,sgebbie/ponyc,sgebbie/ponyc,dipinhora/ponyc,kulibali/ponyc,kulibali/ponyc,sgebbie/ponyc,dipinhora/ponyc,dipinhora/ponyc,jemc/ponyc,jemc/ponyc