shell guide: remove event designator "!?"

[wongsingfo]

The "!?" in the example code will be replaced by the most recent command. It is not a desired behavior.

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[wongsingfo]

@googlebot I signed it!

[IsaacG]

Thanks for the PR! Unfortunately, the authoritative source of this guide for us is not on Github. We will apply this fix internally and push it out.

Sorry for the trouble.

[IsaacG]

This PR was done via #549

Thanks again for the fix!