Ran gofmt on backend.go

I ran gofmt on backend.go, this appears to valid edit.

internal/cache/backend.go

@@ -44,8 +44,8 @@ func (b *Backend) Remove(ctx context.Context, h restic.Handle) error {
 }
 
 var autoCacheTypes = map[restic.FileType]struct{}{
-	restic.IndexFile:    struct{}{},
-	restic.SnapshotFile: struct{}{},
+	restic.IndexFile:    {},
+	restic.SnapshotFile: {},
 }
 
 // Save stores a new file in the backend and the cache.