doc: fix links in synopsis.md

I was looking at synopsis.md and noticed that the links in the file are broken. Basically, they point to .html files instead of .md files. This PR fixes it.

Testing: In the node repository's synopsis.md:

1. Click the "Command Line Options" link. Notice it takes you to a 404 page.
2. Click the "web server" link. Notice it takes you to a 404 page.

In my branch's synopsis.md

1. Click the "Command Line Options" link. Notice it takes you to the correct page.
2. Click the "web server" link. Notice it takes you to the correct page.

Checklist

• make lint
• commit message follows commit guidelines