fix example (#132)

In a real-world usage scenario the user will be importing golang-lru, not running it in the same file. Thus we can't just do `New`, we need to do `lru.New`. While I was at it I changed the example to be a complete runnable example. 

https://replit.com/@almenon/FrenchFickleOutliers#main.go

Co-authored-by: Jeff Mitchell <jeffrey.mitchell@gmail.com>

README.md

@@ -15,11 +15,20 @@ Example
 Using the LRU is very simple:
 
 ```go
-l, _ := New[int, any](128)
-for i := 0; i < 256; i++ {
-    l.Add(i, nil)
-}
-if l.Len() != 128 {
-    panic(fmt.Sprintf("bad len: %v", l.Len()))
+package main
+
+import (
+	"fmt"
+	"github.com/hashicorp/golang-lru/v2"
+)
+
+func main() {
+  l, _ := lru.New[int, any](128)
+  for i := 0; i < 256; i++ {
+      l.Add(i, nil)
+  }
+  if l.Len() != 128 {
+      panic(fmt.Sprintf("bad len: %v", l.Len()))
+  }
 }
 ```