[Book][Validation] Fix small typos

[iamdto]

Q	A
Doc fix?	yes
New docs?	no
Applies to	2.2+
Fixed tickets	-

This fixes syntax highlighting and remove a trailing whitespace on the validation book.

[xabbuh]

Good catch. But unfortunately there's already a PR (see #3345).

[iamdto]

Yeah you're right, I didn't see this one.

@xabbuh @wouterj @weaverryan: maybe PRs and issues titles should be prefixed by something like the first two levels of the hierarchy (i.e. [Cookbook][Assetic], [Book][Assetic] etc.) in order to avoid duplicate PRs?

For example the documentation for contributing to the core code state that:

To ease the core team work, always include the modified components in your
pull request message, like in:

    [Yaml] fixed something
    [Form] [Validator] [FrameworkBundle] added something

But there is no such thing for the documentation.