fix: Added "actuatorgravcomp" to schema.xml

PiperOrigin-RevId: 673294638
Change-Id: Ic92b419097be1449b5a173fe8978897e13a31bb9

Author: Sam Holt

dm_control/mjcf/schema.xml

@@ -1417,6 +1417,7 @@
                 <attribute name="damping" type="float"/>
                 <attribute name="frictionloss" type="float"/>
                 <attribute name="user" type="array" array_type="float"/>
+                <attribute name="actuatorgravcomp" type="keyword" valid_values="false true"/>
               </attributes>
             </element>
             <element name="freejoint" on_demand="true" namespace="joint">