Remove mention about Awaitable not resolvable with itself

This is especially problematic to enforce if we get circular resolutions (i.e. Awaitable 1 is resolved with Awaitable 2 which in turn is resolved with Awaitable 1). In this case it is better to just outright rely on implicit behavior (i.e. recursion).

README.md

@@ -74,7 +74,7 @@ Any implementation MUST at least provide these two parameters. The implementatio
 
 All registered callbacks MUST be executed in the order they were registered. If one of the callbacks throws an `Exception` or `Throwable`, it MUST be rethrown in a callable passed to `Loop::defer` so `Loop::onError` can be properly invoked by the loop. `Loop` refers to the [global event loop accessor](https://github.com/async-interop/event-loop/blob/master/src/Loop.php). The `Awaitable` implementation MUST then continue to call the remaining callbacks with the original parameters.
 
-If an `Awaitable` is resolved with another `Awaitable`, the `Awaitable` MUST keep in pending state until the passed `Awaitable` is resolved. Thus, the value of an `Awaitable` can never be an `Awaitable`. An `Awaitable` MUST NOT be resolved with itself.
+If an `Awaitable` is resolved with another `Awaitable`, the `Awaitable` MUST keep in pending state until the passed `Awaitable` is resolved. Thus, the value of an `Awaitable` can never be an `Awaitable`.
 
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