docs: Make the "always save prime numbers" example more clear

[Tobbe]

Description

Update the "Always save cache" example to make it clear that it's talking about prime numbers

Motivation and Context

I was dropped into that docs page without much context from our GitHub actions CI output

Warning: Input 'save-always' has been deprecated with message: save-always does not work as intended and will be removed in a future release.
A separate `actions/cache/restore` step should be used instead.
See https://github.com/actions/cache/tree/main/save#always-save-cache for more details.

And when I just looked at the example it wasn't immediately clear to me what "Primes" was. If it was "primes" as in "This action 'primes' the cache" or if it was "primes" as in "prime numbers".
Reading it through a few times I'm guessing it's the latter; prime numbers. So this PR updates the docs to remove the ambiguity.

How Has This Been Tested?

Not tested

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[joshmgross]

Thanks!

I can definitely see the confusion between the verb "primes" and the noun "primes" here.