Replace Python with Shell

In the box "Text vs. Whatever" it say Python when should say
Shell.

I move this box two paragraphs up to be closer of the lesson
part about writing the file 'middle.sh'.
Replace "Shell" by "shell"

As point out by @ahmadia we don't use "shell" capitalized (see the
Wikipedia entry: http://en.wikipedia.org/wiki/Shell_(computing)).
Merge pull request #47 from r-gaia-cs/text-processor

Replace Python with Shell