Spelling & more consistent notations in Packaging

* synches -> From a dictionary, it is either syncs or synchs.
I don't like either of them but at least the first one, doesn't seem wrong when I write it.
* `opam` (in backticks) to design the file or the file format (no Opam, and "opam")
* add missing `$ ` prefix to some shell commands
* github, Github -> GitHub, git -> Git, darcs -> Darcs, ...