typo fix to remove duplicated word in sec. 2.3

source/sec-Using_equations.ptx

@@ -71,7 +71,7 @@
 		<p>
 			But wait a minute. The braking distance is just the time it takes from when the driver's foot hits the brake until the car stops. That distance doesn't take into account the driver's reaction time - how long between when the driver thinks to stop and when the driver's foot actually hits the brake. We have a new dependent variable
 			<md>
-				<mrow>D \amp= \text{ total stopping distance distance (feet) } \sim \text{ dep} </mrow>
+				<mrow>D \amp= \text{ total stopping distance (feet) } \sim \text{ dep} </mrow>
 				<mrow>S \amp= \text{ speed (mph) } \sim \text{ indep} </mrow>
 			</md>
 			How can we include this reaction time into an equation? Suppose it takes 1 second to react. We would like to know how many feet that adds to the equation. This is something we can figure out. We know the speed and the time, so multiply them to get the distance, right? One small snag: the speed is in mph (miles per hour). We need to convert units.