docs: update checkout GitHub action version

Using `actions/checkout@v3` produces warnings about deprecated Node.js
16 (see also [1]).

Also, v4 is already used in the "Verifying `rust-version`" example[2].

[1]: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/
[2]: https://github.com/rust-lang/cargo/blob/a9f86addbc3eef731468c3b07addc74a5bc1d8ae/src/doc/src/guide/continuous-integration.md?plain=1#L174

Author: naglis

src/doc/src/guide/continuous-integration.md

@@ -29,7 +29,7 @@ jobs:
           - beta
           - nightly
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}
       - run: cargo build --verbose
       - run: cargo test --verbose
@@ -147,7 +147,7 @@ jobs:
     runs-on: ubuntu-latest
     continue-on-error: true
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: rustup update stable && rustup default stable
       - run: cargo update --verbose
       - run: cargo build --verbose