Update README.md

Fix typo.

README.md

@@ -8,7 +8,7 @@ and test code.
 
 - For a quick overview of Checked C and more information, see our [Wiki](https://github.com/Microsoft/checkedc/wiki).
 - The PDF of the specifcation is available [here](https://github.com/Microsoft/checkedc/releases).
-- Compiler are available [here](https://github.com/Microsoft/checkedc-clang/releases).
+- Compilers are available [here](https://github.com/Microsoft/checkedc-clang/releases).
 
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