Update src/main/java/org/bukkit/material/WoodenStep.java

WoodenStep is a kind of a Step, so its easier to test against (instanceof) Step than Step and WoodenStep.

Btw: I noticed that a inverted wood Step is a org.bukkit.material.Step with the Material "DOUBLE_STEP" while a "normal" step is a "org.bukkit.material.WoodenStep" and "WOOD_STEP".