Fix mistake in Ch7 example code

[nathancarter]

Before running this course again, fix this mistake in the Ch7 example code:

In Section 7.3.1, the example non-DRY code has two parameters (the df and the suffix) but then the example DRY versions of it (several) ignore the suffix parameter. They are not equivalent to the original. It is not trivial to fix this example, because the loop version makes it hard to use multiple parameters without making the example unnecessarily complex.