Update document for pool.download_trusted_certificates

Signed-off-by: Ming Lu <ming.lu@cloud.com>

Author: minglumlu

doc/content/design/external-auth-ldaps.md

@@ -269,7 +269,7 @@ client-->>user: precheck failed
 end
 
 Note over client,coor: sync all ldaps certs
-client->>coor: pool.download_trusted_certificate
+client->>coor: pool.download_trusted_certificates
 coor-->>client:
 client->>join: pool.install_trusted_certificate
 join-->>client:
@@ -294,7 +294,7 @@ client-->>user: pool.join succeed
    - b. if no LDAPS certs, find all `general` certs
 2. Client find all certs in joining host as `certs_joining_host`
 3. Client identify the certs needs to be synced to joining host as `certs_to_sync = certs_pool - certs_joining_host` (certs in `certs_pool`, but not in `certs_joining_host`), the certs fingerprint should be used to identify the certs
-4. Client download all `certs_to_sync`, `pool.download_trusted_certificate` from coordinator
+4. Client download all `certs_to_sync`, `pool.download_trusted_certificates` from coordinator
 5. Client upload all certs to joining pool, `pool.install_trusted_certificate` to joining pool, with the same purpose
 6. Client trigger `pool.join` again with domain username and password
 7. After pool.join: