Changed to plural form

doc/examples/gp_symbreg.rst

@@ -12,7 +12,7 @@ therefore makes a good introductory example for the GP framework in DEAP.
 
 All symbolic regression problems use an arbitrary data distribution, and try
 to fit the most accurately the data with a symbolic formula. Usually, a
-measure like the RMSE (Root Mean Square Error) or MSE (Mean Squared Error) is used to measure an
+measures like the RMSE (Root Mean Square Error) or MSE (Mean Squared Error) are used to measure an
 individual's fitness.
 
 In this example, we use a classical distribution, the quartic polynomial