Fix typo: add missing word “on”

index.html

@@ -143,7 +143,7 @@ module<span class="token punctuation">.</span><span class="token function-variab
 <span class="token keyword">EOF</span></code>
 <code>site <span class="token string">basic-chat</span></code></pre>
 
-      <p>To test it, open up two web browser windows and enter the following into the web developer (JavaScript) console both of them:</p>
+      <p>To test it, open up two web browser windows and enter the following into the web developer (JavaScript) console on both of them:</p>
 
 <pre><code class='language-js'><span class="token keyword">const</span> webSocket <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">WebSocket</span><span class="token punctuation">(</span><span class="token string">'wss://localhost/chat'</span><span class="token punctuation">)</span>
 webSocket<span class="token punctuation">.</span><span class="token function-variable function">onmessage</span> <span class="token operator">=</span> console<span class="token punctuation">.</span><span class="token function">log</span>