`req.params` docs claim that this property can be an array

### Description When 4.x documentation was created the docs were copied from 3.x. One of the breaking changes was [making `req.params` an object](https://github.com/expressjs/express/blob/4453d83ccaed5c80f0c10a23d01216eff612ee56/History.md?plain=1#L1707). This was partially corrected in #134, but the documentation still claims that if a regular expression is used, `req.params` is an array: https://github.com/expressjs/expressjs.com/blob/738f860585a6f34511ba2b2da40c5bd76ef49579/_includes/api/en/4x/req-params.md?plain=1#L11 This isn't a huge problem, since values captured by unnamed capture groups have keys `'0'`, `'1'`, `'2'` etc. and `req.params[n]` access (where `n` is an integer), due to how JS works, will work. **EDIT:** In practice V8 [optimizes](https://v8.dev/blog/fast-properties#named-properties-vs.-elements) array-indexed property access, so I won't mention this in my PR. This error is present both in 4.x and 5.x docs (and [DefinitelyTyped](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/c030728c3e25d15bcb3fbf48edf2df784b9f08db/types/express-serve-static-core/index.d.ts#L383-L388) - looks like this caused some problems, because they trusted the docs https://github.com/DefinitelyTyped/DefinitelyTyped/pull/37502#issuecomment-522155668). ### Expectations API docs (4.x and 5.x) should clearly explain that `req.params` always is an object and how keys are assigned.