Change Spring Boot CLI init command docs default build system to gradle

See gh-40603

spring-boot-project/spring-boot-tools/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/init/InitCommand.java

@@ -165,7 +165,7 @@ private void projectGenerationOptions() {
 			this.packaging = option(Arrays.asList("packaging", "p"), "Project packaging (for example 'jar')")
 				.withRequiredArg();
 			this.build = option("build", "Build system to use (for example 'maven' or 'gradle')").withRequiredArg()
-				.defaultsTo("maven");
+				.defaultsTo("gradle");
 			this.format = option("format", "Format of the generated content (for example 'build' for a build file, "
 					+ "'project' for a project archive)")
 				.withRequiredArg()