Fix the SAM Slack Community link

[rubyrose-alexis]

Issue #, if available:

#1163

Description of changes:

Found the link while reading the documentation of SAM

Description of how you validated changes:

Clicked and link and signed myself up for getting a Slack invite.

Checklist:

• [N/A] Write/update tests
• [N/A] make pr passes
• [Y] Update documentation
• [N/A] Verify transformed template deploys and application functions as expected
• [N/A] Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #1318 into master will decrease coverage by 0.7%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1318      +/-   ##
==========================================
- Coverage    95.3%   94.59%   -0.71%     
==========================================
  Files          78       77       -1     
  Lines        5171     4129    -1042     
  Branches     1190      815     -375     
==========================================
- Hits         4928     3906    -1022     
+ Misses        113      108       -5     
+ Partials      130      115      -15

Impacted Files	Coverage Δ
samtranslator/model/iam.py	81.25% <0%> (-4.47%)	⬇️
samtranslator/swagger/swagger.py	92.57% <0%> (-2.91%)	⬇️
samtranslator/plugins/api/implicit_api_plugin.py	95.57% <0%> (-2.66%)	⬇️
samtranslator/plugins/exceptions.py	83.33% <0%> (-2.39%)	⬇️
samtranslator/model/eventsources/push.py	90.61% <0%> (-1.43%)	⬇️
samtranslator/plugins/globals/globals.py	99.02% <0%> (-0.98%)	⬇️
samtranslator/model/api/api_generator.py	96.72% <0%> (-0.92%)	⬇️
...el/preferences/deployment_preference_collection.py	91.13% <0%> (-0.91%)	⬇️
samtranslator/open_api/open_api.py	93.56% <0%> (-0.83%)	⬇️
samtranslator/model/apigateway.py	97.79% <0%> (-0.66%)	⬇️
... and 21 more

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[keetonian]

Thanks for bringing this up! I just generated a new link and included it in the PR. The splashthat link was a manual process that we've moved off of in recent months.