Fix invalid variable name in input command docs

[holly-hacker]

Description

This PR fixes invalid syntax in the example of the input command. Previously, it contained a variable name user-input, which is not valid in nu 0.69.1. Related to nushell/nushell.github.io#635.

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass

I didn't change any code but ran the test suite anyway and got the following test failure. Seems like this test is not locale aware? :)

failures:

---- conversions::into::string::test::test_examples stdout ----
input: 5 | into string -d 3
result: String { val: "5,000", span: Span { start: 4, end: 15 } }
done: 757.9µs
thread 'conversions::into::string::test::test_examples' panicked at 'the example result is different to expected value: String { val: "5,000", span: Span { start: 4, end: 15 } } != String { val: "5.000", span: Span { start: 0, end: 0 } }', crates\nu-command\src\example_test.rs:140:25
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    conversions::into::string::test::test_examples

[fdncred]

thanks