Increase discoverability of "flip-flop" meaning

Some programmers might not be familiar with what a flip-flop is or what
it does. Further, it can be tricky (but not impossible) to learn more
by searching for "Ruby flip-flop" without being sidetracked into
summertime beach footwear options.

Adding the word "operator" helps narrow down internet searches, and
providing a link to the relevant Wikipedia page allows readers to
easily learn more. At the time of this commit, the Wikipedia page
discusses flip-flop operators within Ruby specifically while also
giving a good overview of why it is good practice to avoid using them.

When a style guide entry advises against using a particular pattern
without explaining what the pattern is, the reader might not be
confident that they can successfully follow the guideline. After all,
it is difficult to avoid doing something when you don't understand what
the "something" is.

README.adoc

@@ -5577,7 +5577,7 @@ Fix by either:
 
 === No Flip-flops [[no-flip-flops]]
 
-Avoid the use of flip-flops.
+Avoid the use of https://en.wikipedia.org/wiki/Flip-flop_(programming)[flip-flop operators].
 
 === No non-`nil` Checks [[no-non-nil-checks]]