fix inlined asciidoc code in en/books/fdp-primer/asciidoc-primer

[vladlen]

Summary

Asciidoc primer in FDP primer contains example, how to include images
into documentation and website. This code rendered wrong in both
html and pdf EN versions, and create troubles to generate correct translated
documents too.

1. "ifdef::" were included to demonstrate real code from articles/releng,

but they are not rendered, but executed in adoc code - they are not shown as it was expected,

2. due to they execution, these "ifdef::" hide this code in pdf version (condition of ifdef:: is false),

and pdf documents does not have text is this example.

3. this kind of AsciiDoc examples do not correctly processed in PO files and require

manual processing after translation.

4. these ifdef:: are not matched, 1 endif:: is missing, it is additional problem in this code.

This ifdef:: are not part of explanation about image directive (they are only nearby code)
and should be deleted from code.

[bcr]

Looks good. We can try to fix the underlying issue separately.

[vladlen]

In D52710#1204585, @bcr wrote:

Looks good. We can try to fix the underlying issue separately.

@bcr I have only doc/ru committers bit. It would be good if someone with EN doc committers bit fix it. I could do it, if I had permission.
It really fixes very annoying problem in this place of documentation.

[maxim]

Taking into account @bcr review, please go ahead and commit with

Reviewed by: bcr
Approved by: maxim (mentor)

[vladlen]

In D52710#1204585, @bcr wrote:

Looks good. We can try to fix the underlying issue separately.

@bcr Similar patch in other fdp-primer file https://reviews.freebsd.org/D52678