fix typos and build warning

- correct some double backslashes
- correct wrong indentation
- correct missing backtick
rephrase sentence on Xml::build()
Merge pull request #1718 from antograssiot/3.0

3.0 fix typos and build warning
finish review of fr cakephp-overview
[fr] review of cakephp-overview
[fr] fix typo
[fr] change en links with fr links in what-is-cakephp
Merge pull request #1720 from cakephp/3.0-fr-overview

3.0 [fr] overview chapter
update fr migration guide
ends review of appendices
[fr] - review migration guide