Update swift-markdown from caafc56d to 7a7c59d.

Updates swift-markdown to include:

    commit 7a7c59d1160fddd23e2379ec5ee8c71df7fd0b3b
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Fri May 20 18:53:24 2022 -0700

	    Improve performance of `Markup.child(at:)` method (swiftlang#44)

	    Improves the performance of `Markup.child(at:)` by refactoring to removing
	    the need to iterate over all previous elements in the child array.

    commit 36cf89e36e94b025dbe37d82fe2a7da3f39d4204
    Author: Franklin Schrans <fschrans@apple.com>
    Date:   Wed Mar 23 14:38:18 2022 +0100

	    Remove extraneous print in test (swiftlang#33)

    commit e50693584310a9190071c96c839485b6f2376832
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Mon Mar 21 10:42:02 2022 -0700

	    Publish Swift Markdown's documentation to GitHub pages (swiftlang#32)

	    * Adopt the Swift-DocC Plugin for documentation generation

	    * Add a script for publishing docs to GitHub pages

	    * Add missing license headers

	    * Move README docs to articles in the DocC catalog

	    * Remove out-of-date documentation about `DiagnosticEngine`

	    Swift markdown no longer includes a DiagnosticEngine so these links were failing
	    to resolve.

    commit 5f10cfb030f43222a49ea34857ff07f02f1e38b0
    Author: christopherweems <github@christopherweems.com>
    Date:   Sat Mar 19 12:53:31 2022 -0400

	    Fix typo in documentation for `MarkupVisitor` (swiftlang#26)

	    Co-authored-by: Christopher Weems <hello@christopherweems.com>

Package.resolved

@@ -60,7 +60,7 @@
         "repositoryURL": "https://github.com/apple/swift-markdown.git",
         "state": {
           "branch": "main",
-          "revision": "caafc56d3794a08c2203fe417b3aff81e2ab2fc1",
+          "revision": "7a7c59d1160fddd23e2379ec5ee8c71df7fd0b3b",
           "version": null
         }
       },