Docs(README): Fix 3 internal link issues

[Noitcereon]

The problem: clicking the links in the README.md on GitHub nor in my local repo with visual studio code did not redirect to the appropriate section when clicked.

The solution: use hyphen (-) instead of underscore(_) in the links

Note: I also briefly tried to fix 2 other links, but could not get them to work in the same way... The links in question:

-  [Problem is that installer gives message that sqldf is not
    available](#problem_is_that_installer_gives_message_that_sqldf_is_not_availa)
-   [Problem with no argument form of sqldf -
    sqldf()](#problem_with_no_argument_form_of_sqldf_-_sqldf())