fix(docs): Tweak label of left-hand positional hold-tap example behavior

We see folks copying this behavior node and duplicating it for the right hand,
then forgetting to modify the "label" value and getting confusing runtime
behavior. If we modify the label to be left-specific like this it might be
a better hint to change it when duplicating it.

docs/docs/behaviors/hold-tap.md

@@ -115,7 +115,7 @@ The following are suggested hold-tap configurations that work well with home row
 	behaviors {
 		lh_pht: left_hand_positional_hold_tap {
 			compatible = "zmk,behavior-hold-tap";
-			label = "POSITIONAL_HOLD_TAP";
+			label = "LEFT_POSITIONAL_HOLD_TAP";
 			#binding-cells = <2>;
 			flavor = "tap-unless-interrupted";
 			tapping-term-ms = <100>;                        // <---[[produces tap if held longer than tapping-term-ms]]