fix: default mail type invalid in .env.example (#2628)

api/.env.example

@@ -82,7 +82,7 @@ UPLOAD_IMAGE_FILE_SIZE_LIMIT=10
 MULTIMODAL_SEND_IMAGE_FORMAT=base64
 
 # Mail configuration, support: resend, smtp
-MAIL_TYPE=resend
+MAIL_TYPE=
 MAIL_DEFAULT_SEND_FROM=no-reply <no-reply@dify.ai>
 RESEND_API_KEY=
 RESEND_API_URL=https://api.resend.com
@@ -131,4 +131,4 @@ UNSTRUCTURED_API_URL=
 SSRF_PROXY_HTTP_URL=
 SSRF_PROXY_HTTPS_URL=
 
-BATCH_UPLOAD_LIMIT=10
+BATCH_UPLOAD_LIMIT=10