remove symbols and list constants from the doc

[LuKuangChen]

Quoted from the doc:

@subsection[#:tag "expressions"]{Expressions}

The base expression values are numbers, strings, symbols,
Booleans. The language also permits, but does not provide useful
operations to work with, list constants, vector constants, and more
exotic quoted forms. (If you don't know what these are, ignore them.)

Because we don't have quote in smol languages, I believe symbols and list constants are not in provided by the language.