Replace Slack channel with discord in module-ballerina-openapi/Package.md

[LakshanSS]

Purpose

• Replace Slack channel with discord in module-ballerina-openapi/Package.md

Related Issue

• Update Slack channel occurrences with Discord ballerina-library#3463

[codecov]

Codecov Report

Base: 75.57% // Head: 75.57% // No change to project coverage 👍

Coverage data is based on head (feda028) compared to base (93b546d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1107   +/-   ##
=========================================
  Coverage     75.57%   75.57%           
  Complexity     1372     1372           
=========================================
  Files           121      121           
  Lines          9142     9142           
  Branches       1453     1453           
=========================================
  Hits           6909     6909           
  Misses         1686     1686           
  Partials        547      547           

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