Add links to all issues in the 2.6.2, 2.6.1 and 2.6.0 sections of the changelog

[rodrigoprimo]

Description

This PR adds links to all issues in the 2.6.2, 2.6.1 and 2.6.0 sections of the changelog and fixes a typo.

Version 2.6.2 includes a link to the PEAR issue tracker. It seems that PHPCS issues reported in PEAR start with the ID 8834 (https://pear.php.net/bugs/search.php?cmd=display&package_name[]=PHP_CodeSniffer&status=All), and the greatest issue ID in the old GitHub repository up until now is 3942. So, it seems that the issue number can be used to determine whether a given issue in the changelog is a GitHub or PEAR issue.

Related issues/external references

Part of #244

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)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

Thanks @rodrigoprimo !

So, it seems that the issue number can be used to determine whether a given issue in the changelog is a GitHub or PEAR issue.

Correct and the older the changelog, the more issues you'll find with PEAR references ;-)