fix(lint): correct camelcase rule docs for destructuring and named imports

[bartlomieju]

Addresses denoland/deno_lint#1324.

The camelcase rule docs list two snippets in the Invalid section that
are actually valid:

• const { last_name } = obj1; — shorthand destructuring binds a local whose
  name is fixed by the source object property; the user has no control over it.
• import { not_camelCased } from "external-module.js"; — a named import binds
  the exported name from the external module, likewise outside the user's
  control.

This matches the rule's documented note that it applies to imported/exported
variables but not to object properties of those variables, and reflects the
behaviour change in denoland/deno_lint#1187.

Verified against deno_lint main:

$ deno lint --rule camelcase
const obj1 = { last_name: "x" };       // flagged (object literal key) — stays Invalid
const { last_name } = obj1;            // no diagnostic — Valid
import { not_camelCased } from "...";   // no diagnostic — Valid

Only the object-literal key on line 1 is reported, confirming the two moved
snippets belong in the Valid section.

This supersedes the stale denoland/deno_lint#1325, which targeted the old
in-repo docs/rules/camelcase.md that was since removed when rule docs moved
to this repo.