Remove executable bit from 2 Markdown files

[rgoldberg]

Remove executable bit from 2 Markdown files.

Resolve #116

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

[rgoldberg]

@lorentey Hi. Sorry to contact out of the blue.

I assume that swift-atomics won't receive many (or possibly any) updates in the future given that Synchronization.Atomic is now available.

I, however, can't use that, because I cannot yet require my users to be running macOS 15+.

In the meantime, I must still use swift-atomics. I assume that others are in similar circumstances.

Could you possibly release a new version of swift-atomics that includes all of the post-1.2.0 improvements in main? One of them fixes a build warning that I keep encountering.

Thanks.