Add default for openid_uid_field to .en (#5563)

sample.env

@@ -45,7 +45,7 @@ REDIS_URL=
 #OPENID_CONNECT_CLIENT_SECRET=
 #OPENID_CONNECT_ISSUER=
 #OPENID_CONNECT_REDIRECT=
-#OPENID_CONNECT_UID_FIELD=
+#OPENID_CONNECT_UID_FIELD=sub
 
 # To enable hCaptcha on the user sign up and sign in, define these 2 keys
 # More information: https://docs.bigbluebutton.org/greenlight/v3/install/#hcaptcha-setup