Remove commented-out text that's causing a DocC misparse.

The tildes look like the start of a fenced code block, inverting the
logic in DocC that knows at-directive blocks can't start in a code
listing, which in turn causes the @available in the non-sendable example
to be misparsed as the start of a directive.

Works around swiftlang/swift-markdown#88

Author: amartini51

Sources/TSPL/TSPL.docc/LanguageGuide/Concurrency.md

@@ -643,7 +643,6 @@ see [TaskGroup](https://developer.apple.com/documentation/swift/taskgroup).
   .. _Concurrency_ChildTasks:
   
   Adding Child Tasks to a Task Group
-  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   
   - Creating a group with ``withTaskGroup`` and ``withThrowingTaskGroup``