Replace E-easy with good first issue in CONTRIBUTING.md

`E-easy` isn't used, so `good first issue` is more appropriate.

Author: giraffate

CONTRIBUTING.md

@@ -63,8 +63,8 @@ To figure out how this syntax structure is encoded in the AST, it is recommended
 Usually the lint will end up to be a nested series of matches and ifs, [like so][deep-nesting].
 But we can make it nest-less by using [if_chain] macro, [like this][nest-less].
 
-[`E-medium`] issues are generally pretty easy too, though it's recommended you work on an E-easy issue first.
-They are mostly classified as [`E-medium`], since they might be somewhat involved code wise,
+[`E-medium`] issues are generally pretty easy too, though it's recommended you work on an [`good first issue`]
+first. They are mostly classified as [`E-medium`], since they might be somewhat involved code wise,
 but not difficult per-se.
 
 [`T-middle`] issues can be more involved and require verifying types. The [`ty`] module contains a