doc: use consistent typography in streams.md

In prose, always surround `null`-as-a-value in backticks.

PR-URL: https://github.com/nodejs/node/pull/6986
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>

doc/api/stream.md

@@ -1856,7 +1856,7 @@ pre-v0.10 style streams can be wrapped in a Readable class using the
 There are some cases where it is necessary to trigger a refresh of the
 underlying readable stream mechanisms, without actually consuming any
 data. In such cases, it is possible to call `readable.read(0)`, which will
-always return null.
+always return `null`.
 
 If the internal read buffer is below the `highWaterMark`, and the
 stream is not currently reading, then calling `stream.read(0)` will trigger