Correction: Use null instead of NaN when stationary (#173)

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>