ch03: fix possible typo in "Local Symbols" section

[vsemozhetbyt]

The current variant contradicts the note in the "Symbols" section above:

...local symbols, created with the Symbol built-in wrapper object and accessed by storing a reference or via reflection...

and the possibility of Reflect.ownKeys() to obtain symbol keys.

[bevacqua]

How about pre-ES6 reflection or iteration methods?

[vsemozhetbyt]

If this is a conventional term for pre-ES6 metaprogramming, then maybe)

[vsemozhetbyt]

@bevacqua Should I amend with pre-ES6 reflection or iteration methods?

[bevacqua]

Sure thing!

[vsemozhetbyt]

Hopefully done)