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?
* 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!