Clarify the number of supported argument collections in article which discusses combinatoric parameterized testing (#1210)

This is a small adjustment to the documentation article [Implementing
parameterized
tests](https://swiftpackageindex.com/swiftlang/swift-testing/main/documentation/testing/parameterizedtesting),
in the section [Test with more than one
collection](https://swiftpackageindex.com/swiftlang/swift-testing/main/documentation/testing/parameterizedtesting#Test-with-more-than-one-collection).
It currently has a sentence which ends:

> …, elements from the second collection are passed as the second
argument, **and so forth.**

The "and so forth" has caused some confusion since it implies that you
can pass more than two argument collections, but in reality only two are
supported currently. Eventually this could potentially be expanded with
further improvements to the testing library, but that is tracked
separately and the docs should reflect its current capabilities.

Fixes rdar://154647425

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Sources/Testing/Testing.docc/ParameterizedTesting.md

@@ -146,8 +146,8 @@ func makeLargeOrder(of food: Food, count: Int) async throws {
 ```
 
 Elements from the first collection are passed as the first argument to the test
-function, elements from the second collection are passed as the second argument,
-and so forth.
+function, and elements from the second collection are passed as the second
+argument.
 
 Assuming there are five cases in the `Food` enumeration, this test function
 will, when run, be invoked 500 times (5 x 100) with every possible combination