slapos: use better option name in slapos.cfg.example

both signature-private-key-file and signature_private_key_file works for
slapos.core but only signature-private-key-file works in slapcache and
slapos.libnetworkcache

slapos.cfg.example

@@ -54,8 +54,8 @@ download-binary-dir-url = http://shadir.nxdcdn.com
 #download-dir-url = http://shadir.nxdcdn.com
 
 # Upload Configuration
-#signature_private_key_file = /etc/opt/slapos/shacache/signature.key
-#signature_certificate_file = /etc/opt/slapos/shacache/signature.cert
+#signature-private-key-file = /etc/opt/slapos/shacache/signature.key
+#signature-certificate-file = /etc/opt/slapos/shacache/signature.cert
 #upload-binary-dir-url = https://www.shacache.org/shadir
 #upload-binary-cache-url = https://www.shacache.org/shacache
 #upload-cache-url = https://www.shacache.org/shacache