net: dns_resolver: Fix request-key cross-reference

Link to "Key Request Service" docs uses file:// scheme instead due to
angled brackets markup. Fix it to proper cross-reference.

Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/dns_resolver.rst

···
 dns_query() returns a copy of the value attached to the key, or an error if
 that is indicated instead.
 
-See <file:Documentation/security/keys/request-key.rst> for further
-information about request-key function.
+See Documentation/security/keys/request-key.rst for further information about
+request-key function.
 
 
 Debugging