Update CONTRIBUTING.md to fix fourmolu version to 0.12 (haskell#9094)

CONTRIBUTING.md

@@ -144,7 +144,7 @@ Code Style
 ---------------
 
 We use automated formatting with Fourmolu to enforce a unified style across the code bases. It is checked in the CI process.
-After installing Fourmolu ≥ 0.12, you can automatically format the code bases with `make style` at the top level of the project.
+After installing Fourmolu 0.12, you can automatically format the code bases with `make style` at the top level of the project.
 You can also use `make style-modified` to only format modified files.
 
 Other Conventions