index.html: update link to FM vs. PM explanation

index.html

@@ -1313,7 +1313,7 @@ <h3>Aren't Phase Modulation and Frequency Modulation equivalent? Why have both?<
         always equivalent. They are only identical when the modulator signal is a
         sinusoid, but with each added harmonic, PM and FM start to differ more and
         more. A detailed mathematical explanation of this is shown in
-        <a href="https://github.com/attilammagyar/js80p/blob/main/pm-fm-equivalence.md" target="_blank">here</a>.</p>
+        <a href="https://github.com/attilammagyar/js80p/blob/main/doc/pm-fm-equivalence.pdf" target="_blank">here</a>.</p>
       </div>
 
       <div class="section" id="faq-name">