Updates readme for macOS 12.2.1

[bwiedmann]

Got the following errors upon compilation on macOS 12.2.1:

mp4.cpp:707:44: error: expected expression
        std::sort(packets.begin(), packets.end(), [](const Match &m1, const Match &m2) { return m1.offset < m2.offset; });
                                                  ^
...
mp4.cpp:790:35: error: expected expression
        sort(group.begin(), group.end(), [](const Match &m1, const Match &m2) { return m1.chances > m2.chances; });
...
39 warnings and 2 errors generated.

Was able to fix it by adding -std=c++11 to the compile command.