Fix the dropck doc formatting to avoid hitting four-space indent.

This was causing `rustdoc` to interpret the part starting with
`(A.) ...` as a code block based on its four-space indentation,
which then was treated by `rustdoc` as a *Rust* code snippet,
and thus was attempting (and failing) to parse my english as
Rust code. Thus causing the compiler-docs build to fail.

Independently, we should probably change `rustdoc` to not interpret
four-space indents as code that needs to be tested; it seems too
perilous to me at least.

(But the formatting here needed to be changed either way.)

cc Issue #25699.

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