Remove reference to NaN heading value

Matches specification change in w3c/geolocation#173 and discussion about existing implementations in w3c/geolocation#171.

files/en-us/web/api/geolocationcoordinates/index.md

@@ -25,7 +25,7 @@ _The `GeolocationCoordinates` interface doesn't inherit any properties._
 - {{domxref("GeolocationCoordinates.altitudeAccuracy")}} {{ReadOnlyInline}}
   - : Returns a `double` representing the accuracy of the `altitude` expressed in meters. This value can be `null` if the implementation cannot provide the data.
 - {{domxref("GeolocationCoordinates.heading")}} {{ReadOnlyInline}}
-  - : Returns a `double` representing the direction towards which the device is facing. This value, specified in degrees, indicates how far off from heading true north the device is. `0` degrees represents true north, and the direction is determined clockwise (which means that east is `90` degrees and west is `270` degrees). If `speed` is `0`, `heading` is {{jsxref("NaN")}}. If the device is unable to provide `heading` information, this value is `null`.
+  - : Returns a `double` representing the direction towards which the device is facing. This value, specified in degrees, indicates how far off from heading true north the device is. `0` degrees represents true north, and the direction is determined clockwise (which means that east is `90` degrees and west is `270` degrees). If `speed` is `0` or the device is unable to provide `heading` information, `heading` is `null`.
 - {{domxref("GeolocationCoordinates.speed")}} {{ReadOnlyInline}}
   - : Returns a `double` representing the velocity of the device in meters per second. This value can be `null`.