Fix a minor lint on an error string

I've always felt like the starting capital here was weird (even if
grammatically correct), so I'm not sure why I didn't rewrite it like
this before.

internal/store/bbolt/bbolt.go

@@ -20,17 +20,13 @@ type Store struct {
 // New creates a new store backed by the provided (pre-opened) bbolt database.
 func New(db *bolt.DB, bucket string) (Store, error) {
 	if db == nil {
-		return Store{}, errors.New("Bolt instance is required")
+		return Store{}, errors.New("bolt.DB instance is required")
 	}
-
 	if bucket == "" {
 		return Store{}, errors.New("bucket is required")
 	}
 
-	return Store{
-		db:     db,
-		bucket: bucket,
-	}, nil
+	return Store{db: db, bucket: bucket}, nil
 }
 
 // List obtains the set of stored groups.