docs: Fix HeaderMatchingFilter example

[wentasah]

The example should tag emails from Redmine with two tags: a fixed "redmine" and a dynamic one based on the project name. However, the configuration does something different: It tags ALL emails with the "redmine" tag and additionally tags emails with the "X-Redmine-Project" header with its content. The reason for tagging all emails is that a missing header is considered having an empty value and the pattern matches an empty string.

The fix is to ensure that the pattern does not match an empty string.

[GuillaumeSeren]

Hey, thank you for the patch :+1: