stir_shaken.conf.sample: Fix bad references to private_key_path

They should be private_key_file.

Resolves: #854

configs/samples/stir_shaken.conf.sample

@@ -113,7 +113,7 @@ Example "attestation" object:
 
 ;[attestation]
 ;global_disable = no
-;private_key_path = /var/lib/asterisk/keys/stir_shaken/tns/multi-tns-key.pem
+;private_key_file = /var/lib/asterisk/keys/stir_shaken/tns/multi-tns-key.pem
 ;public_cert_url = https://example.com/tncerts/multi-tns-cert.pem
 ;attest_level = C
 
@@ -176,7 +176,7 @@ Example "tn" object:
 
 ;[18005551515]
 ;type = tn
-;private_key_path = /var/lib/asterisk/keys/stir_shaken/tns/18005551515-key.pem
+;private_key_file = /var/lib/asterisk/keys/stir_shaken/tns/18005551515-key.pem
 ;public_cert_url = https://example.com/tncerts/18005551515-cert.pem
 ;attest_level = C