typos

src/main/java/UserInterface.java

@@ -13,7 +13,7 @@ public class UserInterface {
     public static void main(String args[]) {
         while(!quit) {
             System.out.println("\nALPHA/BETA T-CELL RECEPTOR MATCHING SIMULATOR");
-            System.out.println("Please select and option:");
+            System.out.println("Please select an option:");
             System.out.println("1) Generate a population of distinct cells");
             System.out.println("2) Generate a sample plate of T-cells");
             System.out.println("3) Simulate T-Cell matching");