Use an example for the playground that actually looks good (#345)

playground/index.html

@@ -27,7 +27,7 @@
         <div class="container">
             <h1>LaTeX to MathML</h1>
             <p class="explanation-link">
-                <a href="#what-is-this">What is this?</a>
+                <a href="#what-is-this">What is this?</a> / <a href="https://github.com/tmke8/latex2mmlc">GitHub</a>
             </p>
             <div class="converter">
                 <div id="outputField" class="output-box"></div>
@@ -112,10 +112,9 @@ <h2 id="what-is-this">What is this?</h2>
                     (or, more specifically, MathML Core), which is then rendered
                     by your browser. For example, try this input:
                 </p>
-                <pre class="copyable"><code>\int \mathcal{D}\!x =
-\lim_{N \to \infty}
-\left( \frac{m}{2\pi i \hbar \Delta t} \right)^\frac{N}{2}
-\int\!\cdots\!\int \prod_{i=1}^{N-1} dx_i</code></pre>
+                <pre class="copyable"><code>p\big(z;f(\theta)\big) =
+\sum_{i=1}^K \gamma_i \mathcal{N}\big(z; f_i(\theta), \Sigma_i\big),
+\quad\text{where }\sum_{i=1}^K \gamma_i = 1</code></pre>
                 <p>
                     Recent versions of all the major browsers support MathML
                     Core, so you should be able to see the rendered math.
@@ -142,8 +141,8 @@ <h2 id="what-is-this">What is this?</h2>
                     For example:
                 </p>
                 <pre class="copyable"><code>\begin{aligned}
-      x &= 1 \\
-  y + z &= 2
+      x &amp;= 1 \\
+  y + z &amp;= 2
 \end{aligned}</code></pre>
             </div>
         </div>