[pointprocesses] fixed exercises

pointprocesses/exercises/pointprocesses-1.tex

@@ -55,7 +55,7 @@
     \end{solution}
 
     \newsolutionpage    
-    \part Write a function that illustrated the spike times of the
+    \part Write a function that displays the spike times of the
     first $t_{max}$ seconds in a raster plot. Each spike train is one
     row in the raster plot. Each spike is a vertical line at the time
     of the spike. When appropriate, the function should use milliseconds