[readme] Clarify execute_script option

[dtgay]

To protect my reputation, I will not mention how long it took before I realized execute_script is not a boolean option that causes JavaScript to run. Hopefully this PR clarifies the option a bit. Happy to improve the PR if my update is not completely accurate as to what execute_script will accept.

Thanks for your work on Grover! This is a huge improvement over our previous tool.

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If you're curious, passing true to execute_script produces an error which can really lead you down a rabbit hole if you mistakenly assume it's a boolean:

/Users/david/.rbenv/versions/3.0.4/lib/ruby/gems/3.0.0/gems/grover-1.1.4/lib/grover/processor.rb:92:in `call_js_method': Protocol error (Runtime.callFunctionOn): Given expression does not evaluate to a function (Grover::JavaScript::ProtocolError)

To make matters worse, if you add debugging statements to output the function name it's trying to run, the output is screenshot, not true. In retrospect, I understand why this is now, and the problem is obvious. My error was probably built on a number of my own misunderstandings, and on all the context switching I've had to do lately. Hopefully this prevents others from making the same mistake I did!

[abrom]

Looks great @dtgay 👍 thank you. I've updated your change to also include an example of it's use which should hopefully make it even clearer 😄