Commit 40f5e5dfc60e ("CLI: introduce -fsingle-threaded/
-fno-single-threaded", 2021-12-01) removed the `--single-threaded`
option, but didn't update all mentions of it.

    $ zig version
    0.11.0-dev.1568+c9b957c93
    $ zig build-exe --help | grep single
      -fsingle-threaded         Code assumes there is only one thread
      -fno-single-threaded      Code may not assume there is only one thread
    $ zig build-exe --single-threaded foo.zig
    error: unrecognized parameter: '--single-threaded'