Edit the comments similar to Jekyll variables

[wangkuiyi]

After merging sql-machine-learning/sql-machine-learning.github.io#17, I received the error message from Github.com. This PR is to fix this error.

The page build failed for the `master` branch with the following error:

The variable `{{"n_classes", "3"}` on line 31 in `sqlflow/doc/submitter.md` was not properly closed with `}}`. For more information, see https://help.github.com/en/articles/page-build-failed-tag-not-properly-terminated.

For information on troubleshooting Jekyll see:

  https://help.github.com/articles/troubleshooting-jekyll-builds

[mxz96102]

tokens like “{{...}}” in code or comment need to wrap in {% raw %} ... {% endraw %} to escape liquid template language check. So need to wrap them and build

[tonyyang-svail]

LGTM.