fixed README

Author: Code-Hex

README.md

@@ -20,57 +20,60 @@ go version devel go1.18-c2397905e0 Sat Nov 13 03:33:55 2021 +0000 darwin/arm64
 
 ## Install
 
-   go get github.com/Code-Hex/go-generics-cache
+    $ go get github.com/Code-Hex/go-generics-cache
 
 ## Usage
 
 See also [examples](https://github.com/Code-Hex/go-generics-cache/blob/main/example_test.go)
 
+playground: https://gotipplay.golang.org/p/FXRk6ngYV-s
+
 ```go
 package main
 
 import (
 	"fmt"
+	"time"
 
 	cache "github.com/Code-Hex/go-generics-cache"
 )
 
 func main() {
-    // Create a cache. key as string, value as int.
-    c1 := cache.New[string, int]()
+	// Create a cache. key as string, value as int.
+	c1 := cache.New[string, int]()
 
-    // Sets the value of int. you can set with expiration option.
-    c1.Set("foo", 1, cache.WithExpiration(time.Hour))
+	// Sets the value of int. you can set with expiration option.
+	c1.Set("foo", 1, cache.WithExpiration(time.Hour))
 
-    // the value never expires.
-    c1.Set("bar", 2)
+	// the value never expires.
+	c1.Set("bar", 2)
 
-    foo, ok := c.Get("foo")
+	foo, ok := c1.Get("foo")
 	if ok {
 		fmt.Println(foo) // 1
 	}
 
-    fmt.Println(c.Keys()) // outputs "foo" "bar" may random
+	fmt.Println(c1.Keys()) // outputs "foo" "bar" may random
 
-    // Create a cache. key as int, value as string.
-    c2 := cache.New[int, string]()
-    c2.Set(1, "baz")
-    baz, ok := c.Get(1)
+	// Create a cache. key as int, value as string.
+	c2 := cache.New[int, string]()
+	c2.Set(1, "baz")
+	baz, ok := c2.Get(1)
 	if ok {
 		fmt.Println(baz) // "baz"
 	}
 
 	// Create a cache for Number constraint.. key as string, value as int.
 	nc := cache.NewNumber[string, int]()
-    nc.Set("age", 26)
+	nc.Set("age", 26)
 
-    // This will be compile error, because string is not satisfied cache.Number constraint.
-    // nc := cache.NewNumber[string, string]()
+	// This will be compile error, because string is not satisfied cache.Number constraint.
+	// nc := cache.NewNumber[string, string]()
 
-    incremented, _ := nc.Increment("age", 1)
-    fmt.Println(incremented) // 27
+	incremented, _ := nc.Increment("age", 1)
+	fmt.Println(incremented) // 27
 
-    decremented, _ := nc.Decrement("age", 1)
-    fmt.Println(decremented) // 26
+	decremented, _ := nc.Decrement("age", 1)
+	fmt.Println(decremented) // 26
 }
 ```