fix: fix comment typo (#2220)

Use an instead of 'a' if the following word starts with a vowel sound, e.g. 'an in-memory cache'.

core/collection/cache.go

@@ -26,7 +26,7 @@ type (
 	// CacheOption defines the method to customize a Cache.
 	CacheOption func(cache *Cache)
 
-	// A Cache object is a in-memory cache.
+	// A Cache object is an in-memory cache.
 	Cache struct {
 		name           string
 		lock           sync.Mutex