Update voltage wording to be consistent

drivers/AnalogIn.h

@@ -102,7 +102,7 @@ public:
      *
      * @returns A floating-point value representing the current input voltage, measured in volts.
      */
-    float read_volts();
+    float read_voltage();
 
     /**
      * Sets this AnalogIn instance's reference voltage.

drivers/source/AnalogIn.cpp

@@ -54,7 +54,7 @@ unsigned short AnalogIn::read_u16()
     return ret;
 }
 
-float AnalogIn::read_volts()
+float AnalogIn::read_voltage()
 {
     return (this->read() * this->vref);
 }