doc: firefox: Show how to use PKCS#11 Proxy Module

`p11-kit-proxy.so` makes it able to use multiple PKCS#11 modules and
manage them through p11-kit(8).

Fix indentation while here.

doc/builders/packages/firefox.section.md

···
         Pocket = false;
         Snippets = false;
       };
-       UserMessaging = {
-         ExtensionRecommendations = false;
-         SkipOnboarding = true;
-       };
+      UserMessaging = {
+        ExtensionRecommendations = false;
+        SkipOnboarding = true;
+      };
+      SecurityDevices = {
+        # Use a proxy module rather than `nixpkgs.config.firefox.smartcardSupport = true`
+        "PKCS#11 Proxy Module" = "${pkgs.p11-kit}/lib/p11-kit-proxy.so";
+      };
     };
 
     extraPrefs = ''