Update fitbit spo2 docs

commons/connector/fitbit/fitbit_intraday_spo2.avsc

@@ -6,6 +6,6 @@
   "fields": [
     { "name": "time", "type": "double", "doc": "Device timestamp in UTC (s)." },
     { "name": "timeReceived", "type": "double", "doc": "Time that the data was received from the Fitbit API (seconds since the Unix Epoch)." },
-    { "name": "spo2", "type": "float", "doc": "Percentage value of SpO2 calculated at the specific time."}
+    { "name": "spo2", "type": "float", "doc": "Percentage value of SpO2 in the bloodstream. SpO2 data is collected when the user is in an extended period of sleep (at least 3 hours of quality sleep). Spo2 values are calculated on a 5-minute exponentially-moving average and provided at the end of a period of sleep."}
   ]
 }