use p for identifers for points instead of x

Fixes #61

Author: hughsk

exercises/intro-4/files/box.glsl

@@ -1,10 +1,10 @@
-bool inBox(highp vec2 lo, highp vec2 hi, highp vec2 x) {
-  
-  //Test if the point x is inside the box bounded by [lo, hi]
-  
+bool inBox(highp vec2 lo, highp vec2 hi, highp vec2 p) {
+
+  //Test if the point p is inside the box bounded by [lo, hi]
+
   return false;
 }
 
 
 //Do not change this line or the name of the above function
-#pragma glslify: export(inBox)
+#pragma glslify: export(inBox)