Error strings should not be capitalized

Reported by staticcheck 2022.1.2 (v0.3.2)

See: staticcheck.io

backend/cache/cache.go

@@ -1128,7 +1128,7 @@ func (f *Fs) ListR(ctx context.Context, dir string, callback fs.ListRCallback) (
 				case fs.Directory:
 					_ = f.cache.AddDir(DirectoryFromOriginal(ctx, f, o))
 				default:
-					return fmt.Errorf("Unknown object type %T", entry)
+					return fmt.Errorf("unknown object type %T", entry)
 				}
 			}