airsonic: update context-path directive

This line has been broken since the service file was created. According
to the [spring docs] that Airsonic uses, server.context-path is what
should be here. I've tested this by manually setting it as an extra JVM
option on my machine, and it works.

[spring docs]: https://docs.spring.io/spring-boot/docs/1.4.5.RELEASE/reference/htmlsingle/#common-application-properties

nixos/modules/services/misc/airsonic.nix

@@ -141,7 +141,7 @@ in {
           -Dairsonic.home=${cfg.home} \
           -Dserver.address=${cfg.listenAddress} \
           -Dserver.port=${toString cfg.port} \
-          -Dairsonic.contextPath=${cfg.contextPath} \
+          -Dserver.context-path=${cfg.contextPath} \
           -Djava.awt.headless=true \
           ${optionalString (cfg.virtualHost != null)
             "-Dserver.use-forward-headers=true"} \