chore(ci): fix rustfmt edition to 2018

Author: tottoto

.github/workflows/CI.yml

@@ -43,8 +43,8 @@ jobs:
 
       - name: cargo fmt --check
         run: |
-          if ! rustfmt --check --edition 2021 $(git ls-files '*.rs'); then
-            printf "Please run \`rustfmt --edition 2021 \$(git ls-files '*.rs')\` to fix rustfmt errors.\nSee CONTRIBUTING.md for more details.\n" >&2
+          if ! rustfmt --check --edition 2018 $(git ls-files '*.rs'); then
+            printf "Please run \`rustfmt --edition 2018 \$(git ls-files '*.rs')\` to fix rustfmt errors.\nSee CONTRIBUTING.md for more details.\n" >&2
             exit 1
           fi