Fix typo in project status section (#624)

Corrected typo in README.

Author: ParadaCarleton

README.md

@@ -66,7 +66,7 @@ If you use cargo-mutants or just like the idea you can help it get better:
 
 As of August 2025 this is an actively-maintained spare time project. I expect to make [releases](https://github.com/sourcefrog/cargo-mutants/releases) about every one or two months.
 
-It's very usable at it is and there's room for lots more future improvement, especially in adding new types of mutation.
+It's very usable as it is and there's room for lots more future improvement, especially in adding new types of mutation.
 
 If you try it out on your project, [I'd love to hear back in a github discussion](https://github.com/sourcefrog/cargo-mutants/discussions/categories/general) whether it worked well or what could be better: