[Filesystem] Fix upper-case and lower-case inconsistencies

[javiereguiluz]

In this page: http://symfony.com/doc/master/components/filesystem/introduction.html the name of the component methods are used as the title of each subsection. The problem is that sometimes the title is upper-cased (Copy, Touch, etc.) and sometimes is not (symlink, mirror).

Should we fix these inconsistencies? My opinion is that we should fix them and we should always use the original method name, even if that makes headings look inconsistent with the rest of the documentation.

[wouterj]

I agree (I build that in my pre-alpha review bot too)