Update README.md

Fix syntax goof that broke a link

README.md

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 ## Testing
 
-- [bats_test]:(docs/bats.md) A test runner for the [Bash Automated Testing System](https://github.com/bats-core/bats-core).
+- [bats_test](docs/bats.md): A test runner for the [Bash Automated Testing System](https://github.com/bats-core/bats-core).
 
 ## Generating documentation