docs: stop using deprecated smallvec! macro in README example

[painbaba]

Fixes #518.

What

The README example still showcased the smallvec! macro, which has been deprecated since 2.0.0-alpha.13 (see #445, deprecation notes in src/lib.rs: use SmallVec::from instead). This PR updates the example to the blessed API:

let mut v: SmallVec<i32, 4> = SmallVec::from([1, 2, 3, 4]);

and drops the macro from the use statement. It also removes a line of trailing whitespace that was on the old import line.

Why

New users copy the README first. Advertising a deprecated macro in the flagship example pushes them toward code that emits deprecation warnings on day one, contradicting the crate's own guidance from #445.

Verification

• SmallVec::from([T; M]) is provided by the unconditional impl<T, const N: usize, const M: usize> From<[T; M]> for SmallVec<T, N> (src/lib.rs, line ~2726) - no feature gate, works in no_std core contexts just like the macro did.
• The README block is not a doctest (not include_str!-ed into lib.rs), so no test churn; example semantics are unchanged (4 inline elements, spill on push(5)).

Note / possible follow-up

src/lib.rs line ~1903 still uses smallvec![1, 2, 3] inside a doctested example on as_mut_ptr (that one IS compiled by CI). Left out of this PR to keep it docs-only and scoped to #518 - happy to do a follow-up if useful.

[alejandro-vaz]

is this ready for review

[alejandro-vaz]

it looks like to me

[alejandro-vaz]

AI contributions are not allowed in any @servo repository

https://book.servo.org/contributing/getting-started.html#ai-contributions

thanks for contributing

[alejandro-vaz]

I'm not sure if you're a human being or not, I've received way too many AI PRs recently

if you want you can try #491, it's also docs

thank you for your contribution