Converted var to const if there is no mutation in var.

This is checked from compiler version 0.12.0-dev.1664

exercises/025_errors5.zig

@@ -26,7 +26,7 @@ fn addFive(n: u32) MyNumberError!u32 {
     // This function needs to return any error which might come back from detect().
     // Please use a "try" statement rather than a "catch".
     //
-    var x = detect(n);
+    const x = detect(n);
 
     return x + 5;
 }