doc: some fixes in repl.md

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, repl

Description of change

1. var => let / const.

2. White space unification (add an infix space in an argument list; change > into > (with a trailing space) in code bits and output examples; explicitly clarify that default REPL prompt contains a trailing space).

3. Fix an output example (make _ reassignment example match with the current output; extend the example for more clarity).

4. Fix a function name (eval => myEval to not shadow the global eval).

5. Replace anonymous functions with an object shorthand and an arrow function.

6. Add the valid link for curl(1) (The current autoinserted link leads to 404 page).

I am sorry I've messed up the previous PR by an inexperienced rebasing during resolving a conflict. Excuse me for wasting your time.