Update xiaomi_aqara.markdown (#11860)

François LASSERRE · frenck · commit 87d2e493a95d · 2020-01-25T14:02:09.000+01:00
Add a line for one gateway don't use mac address because it's doesn't works.
diff --git a/source/_integrations/xiaomi_aqara.markdown b/source/_integrations/xiaomi_aqara.markdown
@@ -68,6 +68,8 @@ xiaomi_aqara:
     - key: xxxxxxxxxxxxxxxx
 ```
 
+For one gateway to avoid errors, don't provide the MAC address.
+
 ### Multiple Gateways
 
 ```yaml
