Use bool instead of boolean in examples

In the Arduino IDE, boolean is an alias for bool.
The Arduino IDE might deprecate boolean in the future, see
https://github.com/arduino/Arduino/issues/4673 for details.

examples/PHSensor/PHSensor.ino

@@ -36,8 +36,8 @@
 
 unsigned long SLEEP_TIME = 60000; // Sleep time between reads (in milliseconds)
 float lastPH;
-boolean receivedConfig = false;
-boolean metric = true;
+bool receivedConfig = false;
+bool metric = true;
 // Initialize PH message
 MyMessage msg(0, V_PH);