man: clarify that /etc/verity.d only parses certificates with the .crt extension

man: clarify that /etc/verity.d only parses certificates with the .crt extension

Exposed in the dracut testsuite while adding tests for sysexts:

```
[    2.972948] localhost (sd-merge)[510]: Validation of dm-verity signature failed via the kernel, trying userspace validation instead: Required key not available
[    2.972993] localhost (sd-merge)[510]: Skipping file '/etc/verity.d/dracut.pem', suffix is not '.crt'.
[    2.973045] localhost (sd-merge)[510]: No userspace dm-verity certificates found.
```