fix: Fix typo

Fix `fundmental frequency` to `fundamental frequency`.

Author: Korilakkuma

index.bs

@@ -9727,7 +9727,7 @@ Dictionary {{PeriodicWaveOptions}} Members</h5>
         The {{PeriodicWaveOptions/real}} parameter represents an array of
         <code>cosine</code> terms. The first element (index 0) is the
         DC-offset of the periodic waveform. The second element
-        (index 1) represents the fundmental frequency.  The
+        (index 1) represents the fundamental frequency.  The
         third represents the first overtone and so on.
 </dl>