Fix description of test file to be tests.groovy

Corrected the documentation to refer to file as tests.groovy, rather
than test.groovy. Updated cope snippet as tests.groovy should expect
"Hello World!" rather than "Hello World".

Closes gh-1593

Author: neiljain

spring-boot-docs/src/main/asciidoc/spring-boot-cli.adoc

@@ -220,15 +220,15 @@ In this example, `tests.groovy` contains JUnit `@Test` methods or Spock `Specifi
 classes. All the common framework annotations and static methods should be available to
 you without having to `import` them.
 
-Here is the `test.groovy` file that we used above (with a JUnit test):
+Here is the `tests.groovy` file that we used above (with a JUnit test):
 
 [source,groovy,indent=0]
 ----
 	class ApplicationTests {
 
 		@Test
 		void homeSaysHello() {
-			assertEquals("Hello World", new WebApplication().home())
+			assertEquals("Hello World!", new WebApplication().home())
 		}
 
 	}