Fix spelling mistake in exception thrown in drupal_rewrite_settings()

This issue has novice tasks. If you are an experienced core developer and have multiple commit mentions, please review novices' work on these tasks rather than doing them yourself. Feedback from experienced contributors is valued.

Problem/Motivation

The following exception message has a spelling mistake.

throw new Exception('Unepxected token after replacing value.');

Proposed resolution

Fix it.

Remaining tasks

• Write patch (novice)
• Review patch to check it fixes the issue, the change is properly documented and for coding standards. Provide test evidence (novice)
• Keep issue summary up to date (novice)

User interface changes

None

API changes

None