Update config override documentation

As per https://github.com/ucb-bar/rocket-chip/pull/507

README.md

@@ -423,10 +423,10 @@ post, so please stay tuned.
 To override specific configuration items, such as the number of external interrupts,
 you can create your own Configuration(s) and compose them with Config's ++ operator
 
-    class WithNExtInterrupts extends Config (nExt: Int) {
-      (pname, site, here) => pname match {
-      case (NExtInterrupts => nExt)
-      }
+    class WithNExtInterrupts(nExt: Int) extends Config {
+        (site, here, up) => {
+            case NExtInterrupts => nExt
+        }
     }
     class MyConfig extends Config (new WithNExtInterrupts(16) ++ new DefaultSmallConfig)