Fix check-polkadot (#4341)

Follows #4317

substrate/.gitlab-ci.yml

@@ -367,7 +367,7 @@ check_polkadot:
     - mkdir -p target/debug/wbuild/.cargo
     - echo "paths = [ \"$SUBSTRATE_PATH\" ]" > target/debug/wbuild/.cargo/config
     # package, others are updated along the way.
-    - cargo update -p sp-io
+    - cargo update
     # Check whether Polkadot 'master' branch builds with this Substrate commit.
     - time cargo check
     - cd -