Remove unidiomatic public modifier in Groovy example code (#2238)

The `public` access modifier is quite unidiomatic in Groovy code.

Co-authored-by: Kevin Wittek <kiview@users.noreply.github.com>

docs/examples/spock/redis/src/test/groovy/quickstart/RedisBackedCacheIntTest.groovy

@@ -10,7 +10,7 @@ class RedisBackedCacheIntTest extends Specification {
     private RedisBackedCache underTest
 
     // init {
-    public GenericContainer redis = new GenericContainer<>("redis:5.0.3-alpine")
+    GenericContainer redis = new GenericContainer<>("redis:5.0.3-alpine")
         .withExposedPorts(6379)
     // }