[README] fix typo in name of warning category statuses

README.md

@@ -82,8 +82,8 @@ function defined in `main.c`.
 By default, if a function name does not correspond to a function in the
 program under analysis, Frama-C will be aborted. This can be changed by
 setting the status of warning category `unknown-func` to a suitable value,
-e.g. `-meta-warn-key unknown-func=enable`, to keep a warning when such a
-case occurs, or `-meta-warn-key unknown-func=disable` to silently ignore this
+e.g. `-meta-warn-key unknown-func=active`, to keep a warning when such a
+case occurs, or `-meta-warn-key unknown-func=inactive` to silently ignore this
 fact.
 
 Note that if you disable the warning in order to use the same meta-properties on