doc: fix unit of size argument of readable.read

"GB" refers to 10**9 bytes, whereas the actual limit is 2**30. The
correct unit symbol is "GiB".

Author: tniessen

doc/api/stream.md

@@ -1126,7 +1126,7 @@ buffer will be returned.
 If the `size` argument is not specified, all of the data contained in the
 internal buffer will be returned.
 
-The `size` argument must be less than or equal to 1 GB.
+The `size` argument must be less than or equal to 1 GiB.
 
 The `readable.read()` method should only be called on `Readable` streams
 operating in paused mode. In flowing mode, `readable.read()` is called