doc: surround Buffer with backticks

doc/api/stream.md

@@ -1502,7 +1502,7 @@ data at once, the `writable._writev()` method should be implemented.
 
 If the `decodeStrings` property is explicitly set to `false` in the constructor
 options, then `chunk` will remain the same object that is passed to `.write()`,
-and may be a string rather than a Buffer. This is to support implementations
+and may be a string rather than a `Buffer`. This is to support implementations
 that have an optimized handling for certain string data encodings. In that case,
 the `encoding` argument will indicate the character encoding of the string.
 Otherwise, the `encoding` argument can be safely ignored.