Update swift-markdown from fae172a to 7e70462

Includes the following changes:

    commit 7e704622ef98b3cc9233283b6298ed61806ddb43
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Sat May 21 14:02:22 2022 -0700

        Correctly set `indexInParent` in `Markup.child(at:)` (#45)

        This fixes a recent regression where `Markup.child(at:)` began returning
        markup with incorrect metadata resulting in out-of-bounds errors.

        The `indexInParent` value of a child is unrelated to the parent's
        `indexInParent`. This was missed initially because tests were only
        checking for children of the first item where `indexInParent` would
        be 0. A new test has been added that asserts `Markup.child(at:)` returns
        correct values for children of nested items as well.

    commit 5cc5656732244237066214bc976570f67cfe47b7
    Author: Ethan Kusters <ekusters@apple.com>
    Date:   Fri May 20 18:53:24 2022 -0700

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

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

Author: ethan-kusters

Package.resolved

@@ -60,7 +60,7 @@
         "repositoryURL": "https://github.com/apple/swift-markdown.git",
         "state": {
           "branch": "release/5.7",
-          "revision": "fae172a60b3ed107bd4dc1e732abfec537661150",
+          "revision": "7e704622ef98b3cc9233283b6298ed61806ddb43",
           "version": null
         }
       },