types: fix struct tag typo

Signed-off-by: ayla <[e-mail]>

types/repo.go

···
 type Branch struct {
 	Reference `json:"reference"`
 	Commit    *object.Commit `json:"commit,omitempty"`
-	IsDefault bool           `json:"is_deafult,omitempty"`
+	IsDefault bool           `json:"is_default,omitempty"`
 }
 
 type RepoTagsResponse struct {