Fix documentation in policy templates

Fix documentation of RegisteredProtocolHandlers policy in policy_templates

BUG=419755

Review URL: https://codereview.chromium.org/658513002

Cr-Commit-Position: refs/heads/master@{#299452}

components/policy/resources/policy_templates.json

@@ -2832,7 +2832,7 @@
             'can_be_recommended': True,
             'can_be_mandatory' : False,
           },
-          'example_value': [{'protocol': 'mailto', 'url': 'https://mail.google.com/mail/?extsrc=mailto&url=%s', 'default': 'true'}],
+          'example_value': [{'protocol': 'mailto', 'url': 'https://mail.google.com/mail/?extsrc=mailto&url=%s', 'default': True}],
           'id': 268,
           'caption': '''Register protocol handlers''',
           'desc': '''Allows you to register a list of protocol handlers. This can only be a recommended policy. The property |protocol| should be set to the scheme such as 'mailto' and the property |url| should be set to the URL pattern of the application that handles the scheme. The pattern can include a '%s', which if present will be replaced by the handled URL.