[Doc]: dollar_ticks example refers to unused formatter classes

[anntzer]

Documentation Link

https://matplotlib.org/stable/gallery/ticks/dollar_ticks.html

Problem

The intro of the example and the backreferences list state that the example uses a FormatStrFormatter (i.e. using old-style %-formatting), but that is not the case (it uses a {}-format string with implicit formatter construction).
The comment in the code "Use automatic StrMethodFormatter" is technically true, but rather unhelpful as there's no reference to what that formatter is.

Suggested improvement

At least the text and backreferences should be modified, but one may also wonder whether this example is useful as standalone (it may be merged with another formatter example, or need more explanatory text, or rewording of the mention of StrMethodFormatter).