Rollup merge of #143961 - 1c3t3a:correct-exploit-mitigations, r=rcvalle

Correct which exploit mitigations are enabled by default

This was brought up by ``@Noratrieb`` in [#project-exploit-mitigations > Incorrect table in the rustc book](https://rust-lang.zulipchat.com/#narrow/channel/343119-project-exploit-mitigations/topic/Incorrect.20table.20in.20the.20rustc.20book/with/523684203). Thanks! :)

[Rendered](https://github.com/rust-lang/rust/blob/132a47e72316b60e99c3e5fefb9c3a06641138e4/src/doc/rustc/src/exploit-mitigations.md)

r? ``@rcvalle``

Author: matthiaskrgr