doc: add missing punctuation in Web Streams doc

PR-URL: #42672
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: tniessen

doc/api/webstreams.md

@@ -29,7 +29,7 @@ streaming data. It is similar to the Node.js [Streams][] API but emerged later
 and has become the "standard" API for streaming data across many JavaScript
 environments.
 
-There are three primary types of objects
+There are three primary types of objects:
 
 * `ReadableStream` - Represents a source of streaming data.
 * `WritableStream` - Represents a destination for streaming data.