fix: typo prebuilt version 2.6.9

Author: amurciasu

ruby-pipeline/ruby-latest.yaml

@@ -61,7 +61,7 @@ steps:
            '--prebuilt-image=2.6.6=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.6:latest',
            '--prebuilt-image=2.6.7=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.7:latest',
            '--prebuilt-image=2.6.8=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.8:latest',
-           '--prebuilt-image=2.6.9=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.8:latest',
+           '--prebuilt-image=2.6.9=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.6.9:latest',
            '--prebuilt-image=2.7.0=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.7.0:latest',
            '--prebuilt-image=2.7.1=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.7.1:latest',
            '--prebuilt-image=2.7.2=gcr.io/gcp-runtimes/ruby/ubuntu16/prebuilt/ruby-2.7.2:latest',