Apply zig fmt to exercises

exercises/13_while3.zig

@@ -21,11 +21,11 @@ pub fn main() void {
 
     // I want to print every number between 1 and 20 that is NOT
     // divisible by 3 or 5.
-    while (n <= 20) : (n+=1) {
+    while (n <= 20) : (n += 1) {
         // The '%' symbol is the "modulo" operator and it
         // returns the remainder after division.
-        if(n % 3 == 0) ???;
-        if(n % 5 == 0) ???;
+        if (n % 3 == 0) ???;
+        if (n % 5 == 0) ???;
         std.debug.print("{} ", .{n});
     }