Add docs for helpful commands to fix format (flutter#8171)

Author: liyuqian

ci/format.sh

@@ -57,7 +57,7 @@ done
 
 if [[ $FAILED_CHECKS -ne 0 ]]; then
   echo ""
-  echo "ERROR: Some files are formatted incorrectly. To fix, apply diffs above via patch -p0."
+  echo "ERROR: Some files are formatted incorrectly. To fix, run \`./ci/format.sh | patch -p0\` from the flutter/engine/src/flutter directory."
   exit 1
 fi