doc: fix typo in stream doc

PR-URL: https://github.com/nodejs/node/pull/8326
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/stream.md

@@ -83,7 +83,7 @@ used to fill the read buffer).
 Data is buffered in Writable streams when the
 [`writable.write(chunk)`][stream-write] method is called repeatedly. While the
 total size of the internal write buffer is below the threshold set by
-`highWaterMark`, calls to `writable.write()` will return `true`. Once the
+`highWaterMark`, calls to `writable.write()` will return `true`. Once
 the size of the internal buffer reaches or exceeds the `highWaterMark`, `false`
 will be returned.