Tiny grammar fix

Just making it more readable. There was an additional "is"

Author: cjgulledge

app/json/higherorderfunctions.json

@@ -61,7 +61,7 @@
       "postparagraph": ""
     },
     {
-      "preparagraph": "`isInstanceOf` is the same as `instanceof` in java, but in this case the parameter types can be *blanked out* using existential types with is a single underline, since parameter type are unknown at runtime.",
+      "preparagraph": "`isInstanceOf` is the same as `instanceof` in java, but in this case the parameter types can be *blanked out* using existential types with a single underline, since parameter type are unknown at runtime.",
       "code": "def addWithSyntaxSugar(x: Int) = (y:Int) => x + y\n\naddWithSyntaxSugar(1).isInstanceOf[Function1[_, _]] should be(__)",
       "solutions": [
         "true"