Fix exception message in NotThrowAfter

The messages of the exceptions that are raised
if the TimeSpan parameters of NotThrowAfter are
negative claim that the parameters should be
"positive". Changed to "non-negative".

Further change:
* Minor formatting change

Src/FluentAssertions/Specialized/ActionAssertions.cs

@@ -142,17 +142,18 @@ public void NotThrowAfter(TimeSpan waitTime, TimeSpan pollInterval, string becau
             FailIfSubjectIsAsyncVoid();
 
             if(waitTime < TimeSpan.Zero)
-                throw new ArgumentOutOfRangeException(nameof(waitTime), $"The value of {nameof(waitTime)} must be positive.");
+                throw new ArgumentOutOfRangeException(nameof(waitTime), $"The value of {nameof(waitTime)} must be non-negative.");
 
             if(pollInterval < TimeSpan.Zero)
-                throw new ArgumentOutOfRangeException(nameof(pollInterval), $"The value of {nameof(pollInterval)} must be positive.");
+                throw new ArgumentOutOfRangeException(nameof(pollInterval), $"The value of {nameof(pollInterval)} must be non-negative.");
 
             TimeSpan? invocationEndTime = null;
             Exception exception = null;
 
             var watch = Stopwatch.StartNew();
 
-            using(var waiter = new ManualResetEvent(false)) {
+            using(var waiter = new ManualResetEvent(false))
+            {
                 while (invocationEndTime is null || invocationEndTime < waitTime)
                 {
                     exception = InvokeSubjectWithInterception();