Blur out example username belonging to a real user

[CourteousCoder]

In list-of-members-selected-bulk.png, blur out the username jsmith of example member "John Smith".

Why:

• That username belongs to a real GitHub user who, as of this writing, appears to be an active user.
• Obscuring the username does not detract from the helpfulness of the screenshot in its context. In each of its uses, the purpose of the screenshot is to demonstrate how to perform bulk actions on members of a GitHub Organization.

Closes: #42795

What's being changed (if available, include any code snippets, screenshots, or gifs):

Just the one image. I downloaded the PNG currently being used, opened it up in GIMP, and just added a blurring slight mosaic effect over the text to obscure the username. Then I replaced the old image. You can see the before- and after-images in the diff of this PR.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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[Sharra-writes]

@CourteousCoder This looks like a reasonable change to me, and definitely the simplest option, so I'll take it.

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