Fix typo in callbacks concept (#1832)

Author: JoelNiemela

concepts/callbacks/about.md

@@ -11,7 +11,7 @@ function applySideLength(callback) {
 }
 
 // Callback must expect the possible argument from the calling function
-function squareArea(side) {
+function areaOfSquare(side) {
   return side * side;
 }
 

concepts/callbacks/introduction.md

@@ -13,7 +13,7 @@ function applySideLength(callback) {
 }
 
 // Callback must expect the possible argument from the calling function
-function squareArea(side) {
+function areaOfSquare(side) {
   return side * side;
 }