Review of the How-to part

This commit reviews the How-to part of the Spring boot doc, fixing mostly typos.

Here are also additional questions/remarks:

* Section 52.1 shows a different way of formatting the properties with an underscore. Intended? Besides we use an hyphen in the Appendix section, maybe we should be consistent in the guide (even if the relaxed environment supports both)
* Section 52.2 _No matter what you set in the environment, Spring Boot will always load
`application.properties` as described above._ I think that's confusing because we just explained how we could rename the name of the file. It's like `application.properties` would still be loaded if we use a different name or location
* Section 52.3: how do you manage things like ${port:8080} and regular maven filtering resources? Previously in the guide, you mentioned that Maven can filter the resources with a fallback if it was not enabled. I guess that would make sense here to add a section that explains how to configure maven filtering to use other tokens than ${} (or revisit our syntax...). I can craft something if you want
* Section 53.6: I am confused by the _Terminate_ word in the title
* Section 55.1: "select a system" is a bit weird
* Section 56.1: what about two data sources? What will the one boot would take by default?
* Section 57: Not sure I get "is in a server"

Looks good overall!