docs: fix typo in errors.rst

[iamsyh]

Description
added "are" instead of "is" as the word "Exceptions" is plural

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Thank you for sending this PR!

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See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md

[kenjis]

Please fix the test failure.

[datamweb]

Suggested change

	What are Exceptions
	------------------
	What are Exceptions
	-------------------

Apply to pass the test.

[kenjis]

Closed by #8938