Fix typo in README.md and clarify some wording

[darkermango]

Summary

This PR fixes a couple of typos in README.md:

• "it hasn't" --> "it doesn't"
• ", is" --> "QRCoder has ..."
• "supports .NET Framework, .NET Core, .NET Standard and .NET" --> "supports most .NET versions (Framework, Core, Standard)"

And a couple more fixes.
You can see all the changes in the diff.

[codebude]

Hi @darkermango ,

since QRCoder will reach its end of life and the repository will be archived on November 1st, 2025, I’d like to clarify how to proceed with this PR.

Would you prefer me to merge it before the archival, or should I close it instead?
Please let me know what you think – I want to handle your contribution in the way that feels most appropriate to you.

Thanks again for your effort and contribution!

[darkermango]

Hey @codebude, Thanks for replying.

I didn't know QRCoder was going to reach it's EOL, but either way, I think it would make the most sense to merge this PR since this should only make the README more understandable and would have no negative effect on the project even if its going to end support.

But if you think it's the better choice not to, feel free to do so.

Best regards, @darkermango.