Update debconf example (#33397)

'true' needs to be quoted as a string, otherwise it is stored as boolean and ends up getting capitalized to 'True' when presented to debconf
Subsequent install of oracle-java8-installer fails with "oracle-license-v1-1 license could not be presented"

ansible version: 2.3.1.0

Actual: value: true
    debconf-show oracle-java8-installer
    * shared/accepted-oracle-license-v1-1: True

Expected: value: 'true'
    debconf-show oracle-java8-installer
    * shared/accepted-oracle-license-v1-1: true

lib/ansible/modules/system/debconf.py

@@ -73,7 +73,7 @@ EXAMPLES = '''
   debconf:
     name: oracle-java7-installer
     question: shared/accepted-oracle-license-v1-1
-    value: true
+    value: 'true'
     vtype: select
 
 - name: Specifying package you can register/return the list of questions and current values