Correction: Use null instead of NaN when stationary

This ammends the existing correction clarifying the units and conditions
for the heading property. This matches existing implementations which
return null in this scenario.

index.html

@@ -943,10 +943,9 @@ <h2>
                           </dt>
                           <dd>
                             A {{double?}} that represents the heading in
-                            degrees, or `null` if not available, or `NaN` if
-                            the device is stationary. Heading measures the
-                            direction in which the device is moving relative to
-                            true north.
+                            degrees, or `null` if not available or the device
+                            is stationary. Heading measures the direction in
+                            which the device is moving relative to true north.
                           </dd>
                         </dl>
                       </li>