Hi @myVietTutor, that is a forward slash (the backslash is \). But it's the right slash to add here. 
Did you restart Apache after making this change?
If so, could you check whether there is any other Redirect or RewriteRule present anywhere?
For example
grep -i -r Redirect /opt/bitnami/apache2/conf
grep -i -r RewriteRule /opt/bitnami/apache2/conf