fixed typo (dotnet#2383)

Author: mairaw

docs/core/tools/dotnet-test.md

@@ -128,7 +128,7 @@ Expressions can be joined with conditional operators:
 
 You can enclose expressions in paranthesis when using conditional operators (for example, `(Name~TestMethod1) | (Name~TestMethod2)`).
 
-For a additional information and examples on how to use selective unit test filtering, see [Running selective unit tests](../testing/selective-unit-tests.md).
+For additional information and examples on how to use selective unit test filtering, see [Running selective unit tests](../testing/selective-unit-tests.md).
 
 ## See also