[minor] Correct TOC case

Author: stephane-caron

README.md

@@ -106,11 +106,11 @@ Available robot descriptions ([gallery](https://github.com/robot-descriptions/aw
 
 * [Arms](#arms)
 * [Bipeds](#bipeds)
-* [Dual Arms](#dual-arms)
+* [Dual arms](#dual-arms)
 * [Drones](#drones)
 * [Educational](#educational)
-* [End Effectors](#end-effectors)
-* [Mobile Manipulators](#mobile-manipulators)
+* [End effectors](#end-effectors)
+* [Mobile manipulators](#mobile-manipulators)
 * [Humanoids](#humanoids)
 * [Quadrupeds](#quadrupeds)
 * [Wheeled](#wheeled)