config: Update dev configuration default url path

Created by: SunilMohanAdapa

I find that I am almost always during development runs using ./run --debug --no-daemon --server_dir=/plinth . The daemon part is gone due to recent changes. But without specifying --server_dir it will run on / and since it is not the same as production run, it change this to /plinth. I am assuming this also the case with other people. So, change the default to /plinth. I believe this reduces the entry to development barrier by a tiny bit.