docs(router): fix code-example headers (#42507)
docs(router): fix code-example headers (#42507)

Previously, the code-examples headers used in the
"Creating custom route matches" tutorial contained the name of the
example directory (`routing-with-urlmatcher`). This was confusing,
because the user was previously instructed to create an app named
`angular-custom-route-match` and switch to that directory.

This commit fixes it by removing the root directory name from the
headers, thus leaving the path of the file relative to the current
working directory. This also aligns with code-examples in other guides.

PR Close #42507

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