docs: minor fixes and typo

[yagogarea]

PR Description:

• Rename beamexample.erl to beamexample1.erl for consistency.
• In chapter 7.3. Calls there's a reference to Chapter 8 as if it has already been covered, even though it comes later.
• Fix minor typos.
• Add 2> to indicate that observer:start() is Erlang shell command.

[happi]

Thanks