AP_LandingGear: Use SI units conventions in parameter units

Follow the rules from:
http://physics.nist.gov/cuu/Units/units.html
http://physics.nist.gov/cuu/Units/outside.html
and
http://physics.nist.gov/cuu/Units/checklist.html
one further constrain is that only printable (7bit) ASCII characters are allowed

libraries/AP_LandingGear/AP_LandingGear.cpp

@@ -12,7 +12,7 @@ const AP_Param::GroupInfo AP_LandingGear::var_info[] = {
     // @DisplayName: Landing Gear Servo Retracted PWM Value
     // @Description: Servo PWM value when landing gear is retracted
     // @Range: 1000 2000
-    // @Units: pwm
+    // @Units: PWM
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("SERVO_RTRACT", 0, AP_LandingGear, _servo_retract_pwm, AP_LANDINGGEAR_SERVO_RETRACT_PWM_DEFAULT),
@@ -21,7 +21,7 @@ const AP_Param::GroupInfo AP_LandingGear::var_info[] = {
     // @DisplayName: Landing Gear Servo Deployed PWM Value
     // @Description: Servo PWM value when landing gear is deployed
     // @Range: 1000 2000
-    // @Units: pwm
+    // @Units: PWM
     // @Increment: 1
     // @User: Standard
     AP_GROUPINFO("SERVO_DEPLOY", 1, AP_LandingGear, _servo_deploy_pwm, AP_LANDINGGEAR_SERVO_DEPLOY_PWM_DEFAULT),