doc:fix unit test directory name (#1939)

test/TESTING.md

@@ -1,3 +1,3 @@
 ## Testing
 
-Unit test are critical to the OpenZeppelin framework. They help ensure code quality and mitigate against security vulnerabilities. The directory structure within the `/tests` directory corresponds to the `/contracts` directory. OpenZeppelin uses Mocha’s JavaScript testing framework and Chai’s assertion library. To learn more about how to tests are structured, please reference OpenZeppelin’s Testing Guide.
+Unit test are critical to the OpenZeppelin framework. They help ensure code quality and mitigate against security vulnerabilities. The directory structure within the `/test` directory corresponds to the `/contracts` directory. OpenZeppelin uses Mocha’s JavaScript testing framework and Chai’s assertion library. To learn more about how to tests are structured, please reference OpenZeppelin’s Testing Guide.