Replace allocator with validator

[fmauch]

The example for adding a custom validator function was labeled "Here is an example custom allocator."

I assume this is kind of a false friend, as the devs more often are confronted with custom allocators rather than validators ;-)

[tylerjw]

I'm confused; this is documenting a custom validator, not a custom allocator.

[tylerjw]

You were right, we somehow have the wrong word there. Re-opening and merging this. I wonder if spell check did this.