fix: allow whitespace character in license SLUG

This PR fixes the regex for the license SLUG by (1) allowing the space
character to occur and (2) moving the `-` to the beginning of the list
of allowed characters where it belongs.

Signed-off-by: Ingo Müller <[email protected]>

verify-spdx-headers

@@ -4,7 +4,7 @@
 import os
 import re
 
-SLUG = re.compile('[a-zA-Z0-9.-]+')
+SLUG = re.compile('[- a-zA-Z0-9.]+')
 SPDX = re.compile(f'SPDX-License-Identifier:\s+({SLUG.pattern})')
 
 class Language: