add comment about sql injection

src/db/contents.ts

@@ -75,6 +75,7 @@ class KnexContentsAccessor implements ContentAccessor{
                 query = query.from("contents");
             }
             if(word !== undefined){
+                //don't worry about sql injection.
                 query = query.where('title','like',`%${word}%`);
             }
             if(content_type !== undefined){