Fix test_matrix example that does not compile in README

[ollien]

The current example in the README does not compile, I thought I'd adjust to an example that does :)

[~/Documents/code/test-case-example] nick@cubert$ cargo t
   Compiling tmp v0.1.0 (/home/nick/Documents/code/test-case-example)
error[E0428]: the name `_2_4_expects` is defined multiple times
  --> src/lib.rs:9:5
   |
9  | /     #[test_matrix(
10 | |         [-2, 2],
11 | |         [-4, 4]
12 | |     )]
   | |______^ `_2_4_expects` redefined here
   |
   = note: `_2_4_expects` must be defined only once in the value namespace of this module
   = note: this error originates in the attribute macro `test_matrix` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0428`.
error: could not compile `tmp` (lib test) due to previous error