Update README.md, added missing 'a'

README.md

@@ -80,7 +80,7 @@ following notable projects:
 automated test-runner and Graphical User Interface with powerful features for
 Windows and Linux platforms.
 
-[Google Test UI](https://github.com/ospector/gtest-gbar) is test runner that
+[Google Test UI](https://github.com/ospector/gtest-gbar) is a test runner that
 runs your test binary, allows you to track its progress via a progress bar, and
 displays a list of test failures. Clicking on one shows failure text. Google
 Test UI is written in C#.