Remove "pub" specifier from function eat_at_restaurant

[liufeimath]

I'm removing "pub" specifier from function eat_at_restaurant for two reasons:

1. The first time this "pub" appears (Ch 7.3, or source code ch07-03-paths-for-referring-to-an-item-in-the-module-tree.md, line 34), we haven't introduced the keyword "pub" yet. It creates confusions.
2. The "pub" specifier for eat_at_restaurant is not useful or relevant to the whole chapter.

[carols10cents]

Thanks! I've opted to fix this by adding a quick note about what the pub does; you're right that it's not directly relevant to the examples but my intent was to make this as real an example as possible and show a public API function. For the examples that compile, if the pub isn't there, you get an unused code warning, which could be equally confusing.