Fix copy-n-paste error

Author: sbrannen

spring-expression/src/main/java/org/springframework/expression/spel/ExpressionState.java

@@ -93,7 +93,7 @@ public ExpressionState(EvaluationContext context, TypedValue rootObject) {
 
 	public ExpressionState(EvaluationContext context, TypedValue rootObject, SpelParserConfiguration configuration) {
 		Assert.notNull(context, "EvaluationContext must not be null");
-		Assert.notNull(context, "'rootObject' must not be null");
+		Assert.notNull(rootObject, "'rootObject' must not be null");
 		Assert.notNull(configuration, "SpelParserConfiguration must not be null");
 		this.relatedContext = context;
 		this.rootObject = rootObject;