Some fixes to stop confusion of kill delay value

[satetsu888]

After reviewing several previous commits, I discovered that the method in which the kill_delay was specified led to some confusion.

In the initial implementation of kill_delay in commit #49, there was a confusing specification that indicated the value specified as an integer would be parsed as time.Duration and multiplied by time.Second. Therefore, specifying kill_delay as kill_delay = 2 would mean waiting for 2 * 1 second, but specifying kill_delay as kill_delay = "2s" would mean waiting for 2000000000 * 1 second.

In the subsequent commit ff8c2ed, an explanation (# ms) was added to air_example.toml, and a fix was made to multiply by time.Millisecond. Users who refer to air_example.toml can now use this option as expected, but unfortunately, those who execute air init will see a generated config file with kill_delay = "0s".

Then, commit #287 was introduced, and it seemed to aim to help users who configure their kill_delay as "1s". As a result, the configurations that refer to air_example.toml no longer work as expected.

The main topic of this pull request is to clarify the specification for kill_delay to avoid confusion. This involves fixing air_example.toml and some test values with kill_delay. It is now clarified that kill_delay should be a string such as "500ms".

Ideally, the WithArgs method should also be fixed to parse time.Duration, but I still believe this pull request is worthwhile!

[satetsu888]

This pull request is closed because an other solution has been made in #462.