docs(DEVELOPER.md): fix typos on "Tests" section (angular#12029)

Author: imagentleman

DEVELOPER.md

@@ -114,7 +114,7 @@ You should execute the 3 test suites before submitting a PR to github.
 All the tests are executed on our Continuous Integration infrastructure and a PR could only be merged once the tests pass.
 
 - CircleCI fails if your code is not formatted properly,
-- Travis CI fails if any of the test suite describe above fails.
+- Travis CI fails if any of the test suites described above fails.
 
 ## Update the public API tests