fix: Clap error in examples/timestamp

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

`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

examples/timestamp.rs

@@ -7,7 +7,7 @@ fn main() {
         .arg(
             Arg::with_name("verbosity")
                 .short('v')
-                .multiple(true)
+                .multiple_occurrences(true)
                 .help("Increase message verbosity"),
         )
         .arg(