Fix incorrect assert hint in flutter.groovy (#125283)

When I include flutter in my exist android project by
`include_flutter.groovy`, it hint me:
```
Project :app doesn't exist. To custom the host app project name, set `org.gradle.project.flutter.hostAppProjectName=<project-name>` in gradle.properties.
```
But set `org.gradle.project.flutter.hostAppProjectName` has no effect,
acutally need to set `flutter.hostAppProjectName`.

This pr is to fix this incorrect hint.

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packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -1081,7 +1081,7 @@ class FlutterPlugin implements Plugin<Project> {
         // Flutter host module project (Add-to-app).
         String hostAppProjectName = project.rootProject.hasProperty('flutter.hostAppProjectName') ? project.rootProject.property('flutter.hostAppProjectName') : "app"
         Project appProject = project.rootProject.findProject(":${hostAppProjectName}")
-        assert appProject != null : "Project :${hostAppProjectName} doesn't exist. To custom the host app project name, set `org.gradle.project.flutter.hostAppProjectName=<project-name>` in gradle.properties."
+        assert appProject != null : "Project :${hostAppProjectName} doesn't exist. To customize the host app project name, set `flutter.hostAppProjectName=<project-name>` in gradle.properties."
         // Wait for the host app project configuration.
         appProject.afterEvaluate {
             assert appProject.android != null