s/parameter/argument/

README.md

@@ -36,7 +36,7 @@ the tests in progress, however it should almost never be necessary to
 interact with it directly.  With the exception of `run_tests()`
 (described below), all functions provided by the library will take
 take a pointer to the current `TestState` value as their first
-parameter.
+argument.
 
 ## `run_tests()`