Updated docs on running single tests

[modular-magician]

The examples refer to running a single test, but the -run command in the example could potentially run lots of tests unexpectedly.

Use a -run= example with the correct syntax for passing a literal dollar sign to the regex -- while the docs link to the golang docs, because of shell quoting and because of it getting passed via make, it seems to be necessary to use double dollar signs.

I don't 100% know this is the right solution, but I haven't gotten a direct answer or an example of a better way to do it.

Open to suggestions both as to a better fix

I debated anchoring on the front side as well (-run=^TestFoo_bar$$), but seems less necessary. But it is very easy to accidentally kick off a whole bunch of expensive and slow tests if you match too broadly, including if you use a bad regex, forget the dash before run, or do any number of other things.

Fixes #19934

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Derived from GoogleCloudPlatform/magic-modules#12801