Remove confusing reference to “Immediate Undefined Behavior” from docs

[umanwizard]

The docs in core::ptr capitalize "Immediate Undefined Behavior" as if that were a technical term of art; it’s not. The code described just causes undefined behavior, not some special kind that is any more "Immediate" than usual.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from Darksonn, JohnTitor, Mark-Simulacrum

[ChayimFriedman2]

I think the "immediate" is important, as people sometimes tend to think it's only UB when they use the reference. I think if we won't capitalize it it will be all good.

[Mark-Simulacrum]

r=me with a modification to drop the capitalization; there are several other cases of that formulation in the standard library.