E0137 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0137.rs

Error E0137 needs a span_label, updating it from:

error[E0137]: multiple functions with a #[main] attribute
  --> src/test/compile-fail/E0137.rs:17:1
   |
17 | fn f() {} //~ ERROR E0137
   | ^^^^^^^^^

To:

error[E0137]: multiple functions with a #[main] attribute
  --> src/test/compile-fail/E0137.rs:17:1
   |
17 | fn f() {} //~ ERROR E0137
   | ^^^^^^^^^ additional #[main] function

Bonus: add the span_label to show where the first duplicate was created:

error[E0137]: multiple functions with a #[main] attribute
  --> src/test/compile-fail/E0137.rs:17:1
   |
14 | fn foo() {}
   | ----------- first #[main] function
17 | fn f() {} //~ ERROR E0137
   | ^^^^^^^^^ additional #[main] function