Fix PipeOptions ctor pauseWriterThreshold (#4491)

Should be zero instead of negative.

Author: carlossanlop

xml/System.IO.Pipelines/PipeOptions.xml

@@ -52,7 +52,7 @@
         <param name="pool">The pool of memory blocks to be used for buffer management.</param>
         <param name="readerScheduler">The <see cref="T:System.IO.Pipelines.PipeScheduler" /> to be used to execute <see cref="T:System.IO.Pipelines.PipeReader" /> callbacks and async continuations.</param>
         <param name="writerScheduler">The <see cref="T:System.IO.Pipelines.PipeScheduler" /> used to execute <see cref="T:System.IO.Pipelines.PipeWriter" /> callbacks and async continuations.</param>
-        <param name="pauseWriterThreshold">The number of bytes in the <see cref="T:System.IO.Pipelines.Pipe" /> before <see cref="M:System.IO.Pipelines.PipeWriter.FlushAsync(System.Threading.CancellationToken)" /> starts blocking. A negative value prevents <see cref="M:System.IO.Pipelines.PipeWriter.FlushAsync(System.Threading.CancellationToken)" /> from ever blocking, effectively making the number of bytes in the <see cref="T:System.IO.Pipelines.Pipe" /> unlimited.</param>
+        <param name="pauseWriterThreshold">The number of bytes in the <see cref="T:System.IO.Pipelines.Pipe" /> before <see cref="M:System.IO.Pipelines.PipeWriter.FlushAsync(System.Threading.CancellationToken)" /> starts blocking. A value of zero prevents <see cref="M:System.IO.Pipelines.PipeWriter.FlushAsync(System.Threading.CancellationToken)" /> from ever blocking, effectively making the number of bytes in the <see cref="T:System.IO.Pipelines.Pipe" /> unlimited.</param>
         <param name="resumeWriterThreshold">The number of bytes in the <see cref="T:System.IO.Pipelines.Pipe" /> when <see cref="M:System.IO.Pipelines.PipeWriter.FlushAsync(System.Threading.CancellationToken)" /> stops blocking.</param>
         <param name="minimumSegmentSize">The minimum size of the segment requested from <paramref name="pool" />.</param>
         <param name="useSynchronizationContext">