Update README.md (#79)

Fix reference to `IExplicitUsageOnlyValidator`

README.md

@@ -63,7 +63,7 @@ There are some cases though where explicitness is either required or useful, so
 
 For example, if you've got a field argument that's a scalar type (e.g. not an input type), like an `int`, creating
 a validator targeting `int` would mean every top-level `int` argument across your schema would be implicitly validated, which wouldn't make sense.
-Instead, annotate the validator by having it inherit `IExplicitUsageOnly` and then explicitly setup it up on the argument (see below).
+Instead, annotate the validator by having it inherit `IExplicitUsageOnlyValidator` and then explicitly setup it up on the argument (see below).
 
 Annotation API: