E0302 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0302.rs

E0302 needs a span_label, updating it from:

error[E0302]: cannot assign in a pattern guard
  --> src/test/compile-fail/E0302.rs:14:21
   |
14 |         option if { option = None; false } => { }, //~ ERROR E0302
   |                     ^^^^^^^^^^^^^

To:

error[E0302]: cannot assign in a pattern guard
  --> src/test/compile-fail/E0302.rs:14:21
   |
14 |         option if { option = None; false } => { }, //~ ERROR E0302
   |                     ^^^^^^^^^^^^^ assignment in pattern guard