docs: typo (#550)

Author: jackall3n

README.md

@@ -77,7 +77,7 @@ const db = new Low(adapter)
 // Read data from JSON file, this will set db.data content
 await db.read()
 
-// If file.json doesn't exist, db.data will be null
+// If db.json doesn't exist, db.data will be null
 // Set default data
 // db.data = db.data || { posts: [] } // Node < v15.x
 db.data ||= { posts: [] }             // Node >= 15.x