tweak the animated intro

Move the message about self-selected people from the end of Act 1 to the
start of Act 2 and tweak the wording.

Author: foobacca

docs/animated-intro.html

@@ -705,11 +705,6 @@ <h2>The Invitation</h2>
                         For the rest of this walkthrough, we'll use a simplified example: <span class="hl">16 candidates</span>, from which we need to select <span class="hl">8 panellists</span>.
                     </p>
                 </div>
-                <div class="scene">
-                    <div class="key-message">
-                        The candidate pool is <strong>self-selected</strong>. The people who respond are not a random cross-section of the population &mdash; they're the people who <em>chose</em> to respond. This matters.
-                    </div>
-                </div>
             </div>
         </section>
         <!-- ═══════════════════════════════════════════════════ -->
@@ -720,6 +715,12 @@ <h2>The Invitation</h2>
                 <div class="scene">
                     <div class="act-title">Act 2</div>
                     <h2>The Skewed Pool</h2>
+                    <div class="scene">
+                        <div class="key-message">
+                            One key issue we have to address is that the candidate pool is <strong>self-selected</strong>.
+                            The people who respond are not a random cross-section of the population &mdash; they're the people who <em>chose</em> to respond. This matters.
+                        </div>
+                    </div>
                     <p>
                         Here are our 16 candidates. Each has three demographic attributes: <span class="hl">Gender</span>, <span class="hl">Region</span>, and <span class="hl">Age</span>.
                     </p>