Fix error handling in mailer and wiki services (#36041)

- Updated error message in `incoming.go` to remove unnecessary wrapping
of the error.
- Corrected typo in error message in `wiki.go` for clarity.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

services/wiki/wiki.go

@@ -135,7 +135,7 @@ func updateWikiPage(ctx context.Context, doer *user_model.User, repo *repo_model
 	if hasDefaultBranch {
 		if err := gitRepo.ReadTreeToIndex("HEAD"); err != nil {
 			log.Error("Unable to read HEAD tree to index in: %s %v", basePath, err)
-			return fmt.Errorf("fnable to read HEAD tree to index in: %s %w", basePath, err)
+			return fmt.Errorf("unable to read HEAD tree to index in: %s %w", basePath, err)
 		}
 	}