Update README.md to fix broken Slack link

[davidyorke]

Change Slack link that only allows sign-ups for limited domains to generic link that allows general sign-up.

Proposed changes

I arrived at the github page from the freecodecamp youtube video and clicked on the link to get help on slack. When I clicked on the sign up link it prompted me to choose one of four domains allowed. I'm not involved in any of those orgs so I went to file and issue here. When I clicked on "new issue" it listed a lnk to slack that was different from the readme.md page so I clicked it. That one worked.

Fixes # (issue)

Type of change

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• [x ] Bug fix (change which fixes one or more issues)
• New feature (change which adds functionality or features to the course)
• Translations (change which adds or modifies translations of the course)
• Documentation (change which modifies documentation related to the course)
• This change requires an update to the course's z/OS environment

Checklist:

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• [x ] I have read the Contributing Guideline
• [x ] I have included a title and description for this PR
• I have DCO-signed all of my commits that are included in this PR
• I have tested it manually and there are no regressions found
• I have commented my code, particularly in hard-to-understand areas (if appropriate)
• I have made corresponding changes to the documentation (if appropriate)

[MikeBauerCA]

Thanks for opening a PR! The one drawback of using the new link is that it just takes you to the Open Mainframe Project slack (I suspect to the #general channel if it is your first login) and not the #cobol-programming-course channel).

Now that you are signed up for Slack, does the existing link work and direct you to the #cobol-programming-course channel? Perhaps we just need a note such as:
If this is your first time logging into the Open Mainframe Slack, sign up here.

[davidyorke]

@MikeBauerCA the existing link https://openmainframeproject.slack.com/archives/C011NE32Z1T works after being signed-up for the OMP slack and logged-in but I'm having trouble repeating the steps I used to get signed-up. Now when I go to the new URL https://openmainframeproject.slack.com while signed-out and click "create an account" it asks to choose one of the four domains (see attached). Unfortunately, I didn't document the steps I was taking when it suddenly worked for no apparent reason yesterday. I know that it was an e-mail address that I hadn't used for Slack before, I got an e-mail from Slack to verfiy my e-mail address, and then a minute later got a "welcome to OPM slack" e-mail. Another strange thing is that when I click on "home" and "get started" while signed-in it says I was invited by "Tom Slanda" but I don't remember ever requesting such an invite.

I you have any idea of who is administrating the Slack workspace, maybe contact them to find out what the heck is going on with the sign-up page.