Say "group" instead of "multilineGroup" in commands.md (#1921)
Say "group" instead of "multilineGroup" in commands.md (#1921)

`multilineGroup` was removed in commit 3aa267d (https://github.com/prettier/prettier/pull/450)
arijs/prettier-with-tabs,lydell/prettier,azz/prettier,prettier/prettier,rattrayalex/prettier,prettier/prettier,arijs/prettier-miscellaneous,existentialism/prettier,existentialism/prettier,Skywalker13/prettier-space-parenthesis,azz/prettier,existentialism/prettier,prettier/prettier,existentialism/prettier,azz/prettier,lydell/prettier,jlongster/prettier,azz/prettier,arijs/prettier-miscellaneous,prettier/prettier,arijs/prettier-with-tabs,Skywalker13/prettier-space-parenthesis,Skywalker13/prettier-space-parenthesis,lydell/prettier,lydell/prettier,arijs/prettier-miscellaneous,arijs/prettier-miscellaneous,rattrayalex/prettier,rattrayalex/prettier,Skywalker13/prettier-space-parenthesis