Update key for setting the time limit for half measure caching

[jelleroorda]

The documentation for the expiry time for half measure caching seems to be incorrect (outdated?).

You can see this in the ApplicationCacher.php, which inherits it from the parent here: AbstractCacher.php

[jasonvarga]

Oh that's a bummer.

Looks like I wrote the docs how I wanted it to word but never updated the code.

I'd say this is a bug, or something that should be updated in the code itself. I'd rather you define expiry than default_cache_length. There's no "default". It's just what it is.

[jasonvarga]

Closing in favor of statamic/cms#2744