Fixes link to combinatorics.

[powerje]

The link to Combinatorics appears to be wrong, this change updates it to Combinatorics.swift.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've updated the documentation if necessary.

[lorentey]

Oops, thanks for catching this! 👍

[lorentey]

@swift-ci test

[lorentey]

Oh hey, the build got hit by issue #7!

13:53:31 
<unknown>:0: error: fatal error encountered while reading from module 'OrderedCollections'; please file a bug report with your project and the crash log
13:53:31 <unknown>:0: note: module 'OrderedCollections' full misc version is '5.3.2(5.3.2)/Apple Swift version 5.3.2 (swiftlang-1200.0.45 clang-1200.0.32.28)'
13:53:31 
13:53:31 *** DESERIALIZATION FAILURE (please include this section in any bug report) ***
13:53:31 result not found
13:53:31 Cross-reference to module 'OrderedCollections'
13:53:31 ... OrderedSet
13:53:31 ... in an extension in module 'OrderedCollections'
13:53:31 ... Element
13:53:31 
13:53:31 Stack dump:
[...]
13:53:31 1.	Apple Swift version 5.3.2 (swiftlang-1200.0.45 clang-1200.0.32.28)
13:53:31 2.	While deserializing SIL function "$s18OrderedCollections0A3SetV12intersectionyACyxGAEF"
13:53:31 3.	While reading from 'OrderedCollections'
13:53:31 4.	While finishing conformance for protocol conformance to 'Collection' (in module 'Swift') for type 'OrderedSet<Element>'

[lorentey]

@swift-ci test