update nimbus event for sync sign in

https://dictionary.telemetry.mozilla.org/apps/fenix/metrics/sync_auth_sign_in

app/src/androidTest/java/org/mozilla/fenix/ui/NimbusEventTest.kt

@@ -68,7 +68,7 @@ class NimbusEventTest {
         observer.onAuthenticated(mockk(), AuthType.Signin)
 
         Experimentation.withHelper {
-            assertTrue(evalJexl("'sync_auth_sign_in'|eventSum('Days', 28, 0) > 0"))
+            assertTrue(evalJexl("'sync_auth.sign_in'|eventSum('Days', 28, 0) > 0"))
         }
     }
 }

app/src/main/java/org/mozilla/fenix/components/BackgroundServices.kt

@@ -226,7 +226,7 @@ internal class TelemetryAccountObserver(
             // User signed-in into an existing FxA account.
             AuthType.Signin -> {
                 SyncAuth.signIn.record(NoExtras())
-                context.components.analytics.experiments.recordEvent("sync_auth_sign_in")
+                context.components.analytics.experiments.recordEvent("sync_auth.sign_in")
             }
 
             // User created a new FxA account.

app/src/test/java/org/mozilla/fenix/components/BackgroundServicesTest.kt

@@ -145,7 +145,7 @@ class BackgroundServicesTest {
     fun `telemetry account observer records nimbus event for logins`() {
         observer.onAuthenticated(mockk(), AuthType.Signin)
         verify {
-            nimbus.recordEvent("sync_auth_sign_in")
+            nimbus.recordEvent("sync_auth.sign_in")
         }
         confirmVerified(nimbus)
     }