Example find and replace in bash variable

This is part of Issue #54.

In this commit we add one example of find and replace some string
in the value of a bash variable using shell parameter expansion.
Fixed typos suggested by @ahmadia.
Separate sentences in guide-shell-loop

Thanks to @wltrimbl for the catch.
Add @ahmadia suggestions
Merge pull request #58 from r-gaia-cs/teach06-replace

Example find and replace in bash variable