Copy/paste fix in Proxy error message (#1787)

(Really small self-explanatory change - maybe the commit message could be better)

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It changes the following:

When calling `new Proxy(undefined, {})` the error message now refers to the target `undefined` instead of the handler `{}`

boa/src/builtins/proxy/mod.rs

@@ -96,7 +96,7 @@ impl Proxy {
     fn create(target: &JsValue, handler: &JsValue, context: &mut Context) -> JsResult<JsValue> {
         // 1. If Type(target) is not Object, throw a TypeError exception.
         let target = target.as_object().ok_or_else(|| {
-            context.construct_type_error("Proxy constructor called with non-object handler")
+            context.construct_type_error("Proxy constructor called with non-object target")
         })?;
 
         // 2. If Type(handler) is not Object, throw a TypeError exception.