Update mu4e.texi for Fedora command

The currently pescribed installation for webkitgtk3-devel on Fedora is:

    sudo apt-get install webkitgtk3-devel

Instead, it should be:

    sudo yum install webkitgtk3-devel
Merge pull request #328 from robstewart57/patch-1

* update mu4e.texi for Fedora command