correct comment for DB.getEffectiveMem (#150)

diff --git a/leveldb/db_state.go b/leveldb/db_state.go
index b7de09e..590b5e7 100644
--- a/leveldb/db_state.go
+++ b/leveldb/db_state.go
@@ -164,7 +164,7 @@
 	return db.mem, db.frozenMem
 }
 
-// Get frozen memdb.
+// Get effective memdb.
 func (db *DB) getEffectiveMem() *memDB {
 	db.memMu.RLock()
 	defer db.memMu.RUnlock()