CONTRIBUTING.md: use ShellSession code fence

ShellSession gives us marginally better highlighting, comparing

    $ foo
    $ bar

with:

    ```ShellSession
    $ foo
    $ bar
    ```

So, where appropriate, let's convert the former into the later.

CONTRIBUTING.md

@@ -74,31 +74,37 @@ Git LFS depends on having a working Go 1.11.0+ environment.

On RHEL etc. e.g. Red Hat Enterprise Linux Server release 7.2 (Maipo), you will neet the minimum packages installed to build Git LFS:

```
```ShellSession
$ sudo yum install gcc
$ sudo yum install perl-Digest-SHA
```

In order to run the RPM build `rpm/build_rpms.bsh` you will also need to:

`$ sudo yum install ruby-devel`
```ShellSession
$ sudo yum install ruby-devel
```

(note on an AWS instance you may first need to `sudo yum-config-manager --enable rhui-REGION-rhel-server-optional`)

### Building Git LFS

The easiest way to download Git LFS for making changes is `git clone`:

    $ git clone git@github.com:git-lfs/git-lfs.git
    $ cd git-lfs
```ShellSession
$ git clone git@github.com:git-lfs/git-lfs.git
$ cd git-lfs
```

From here, run `make` to build Git LFS in the `./bin` directory. Before
submitting changes, be sure to run the Go tests and the shell integration
tests:

    $ make test          # runs just the Go tests
    $ cd t && make test  # runs the shell tests in ./test
    $ script/cibuild     # runs everything, with verbose debug output
```ShellSession
$ make test          # runs just the Go tests
$ cd t && make test  # runs the shell tests in ./test
$ script/cibuild     # runs everything, with verbose debug output
```

## Updating 3rd party packages