Add template literal "bad" example to single quotes guideline

There has been some confusion around whether we should use single quotes
or template literals. To help avoid this confusion, I am adding a "bad"
example to the single quotes guideline. This rule is already enforced by
the quotes linter rule.

Generally, we want code to clearly express developer intention. Using
template literals communicates that you intend to use some of the
features that template literals offer (e.g. interpolation).

Fixes #992