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.
Changed land to lines

Also added an explanation of how to avoid, since it is a common problem, but feel free to roll that back.
misspelling functiom
mispelled variable name
Revision to exercise prompts.  Chose easier-to-remember variable names and created more empty cells.
Removed is from boolean operators
Simplified knights example ipynb PR #84
Changed to markdown cell, copyedit
Merge pull request #82 from beroe/patch-2

Changed land to lines
Merge pull request #85 from wltrimbl/exerciseprompts

thw-python : Made some of the elementary exercise prompts more explicit; changed values and variable names.
Merge pull request #86 from wltrimbl/removeis

Removed is from boolean operators (thw-python refinements)
Added explicit line describing half-open ranges
Removed spaces before colons per PEP8
Add @ahmadia suggestions
Merge pull request #58 from r-gaia-cs/teach06-replace

Example find and replace in bash variable
Whitespace PEP8
Merge pull request #84 from wltrimbl/knightsrewrite

Simplified knights example to use straightfoward if instead of for...else.
Merge branch 'halfopen' into gh-pages