fix typo

base_classes/NXdata.nxdl.xml

@@ -120,7 +120,7 @@
 			  @y_indices = [0, 1]
 			  data: float[10,20]
 			  x: float[10,20]     --> coordinates along both dimensions
-			  y: float[10,20]     --> coordinates along both dimension
+			  y: float[10,20]     --> coordinates along both dimensions
 
 		In this example each data point ``data[i,j]`` has axis coordinates ``[x[i,j], y[i,j]]``.