Update README

README.md

@@ -31,7 +31,7 @@ go get github.com/marpit19/goquickmap
 Note: All keys in the current implementation must be strings.
 
 ### QuickMap
-```
+```go
 m := quickmap.New()
 m.Insert("key", "value")  // key must be a string
 value, exists := m.Get("key")