Fix broken link in CONTRIBUTING.MD (JuliaLang#25967)

Fix broken link in CONTRIBUTING.MD: https://docs.julialang.org/en/latest/stdlib/Test/ is the new manual page on Unit Testing

CONTRIBUTING.md

@@ -84,7 +84,7 @@ There are never enough tests. Track [code coverage at Coveralls](https://coveral
 
 2. Browse through the source files and find some untested functionality (highlighted in red) that you think you might be able to write a test for.
 
-3. Write a test that exercises this functionality---you can add your test to one of the existing files, or start a new one, whichever seems most appropriate to you. If you're adding a new test file, make sure you include it in the list of tests in `test/choosetests.jl`. https://docs.julialang.org/en/latest/stdlib/test may be helpful in explaining how the testing infrastructure works.
+3. Write a test that exercises this functionality---you can add your test to one of the existing files, or start a new one, whichever seems most appropriate to you. If you're adding a new test file, make sure you include it in the list of tests in `test/choosetests.jl`. https://docs.julialang.org/en/latest/stdlib/Test/ may be helpful in explaining how the testing infrastructure works.
 
 4. Run `make test-all` to rebuild Julia and run your new test(s). If you had to fix a bug or add functionality in `base`, this will ensure that your test passes and that you have not introduced extraneous whitespace.