fix: Clap error in examples/timestamp

[kenoss]

Currently, examples/timestamp raises runtime error by violation of constraints of clap.

$ cargo run --example timestamp
   Compiling stderrlog v0.5.5-alpha.0 (/home/keno/src/github.com/cardoe/stderrlog-rs)
    Finished dev [unoptimized + debuginfo] target(s) in 0.52s
     Running `target/debug/examples/timestamp`
thread 'main' panicked at 'Argument "verbosity"
  Arg::is_takes_value_set is required when Arg::is_multiple_values_set is set.
', /home/keno/.cargo/registry/src/github.heygears.com-1ecc6299db9ec823/clap-3.2.23/src/builder/debug_asserts.rs:746:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Arg::multiple() is deprecated in clap 3.* [1].
This PR uses Arg::multiple_occurrences() instead and fixes the error.

[1] https://docs.rs/clap/3.0.0/clap/struct.Arg.html#method.multiple