Update types.rst

[gdevanla]

Fix a minor typo

[jakebolewski]

Thanks!

[tkelman]

According to the section below, I think this is incorrect, the behavior of get with a default matches what was originally stated - the default value is returned when the firat input is null.

[johnmyleswhite]

This is the opposite of what we do.

[jakebolewski]

I think this can be worded differently to make it clearer. Reading it quickly isolation kind of conflates (in my mind) a Nullable returning type T when the value is !== nothing.

[johnmyleswhite]

If that's confusing, I would use the phrase "null value" instead of "missing value".

[jakebolewski]

I think that would be better.