Add missing keyword: children. (#123)

For YAML inventory files, using filetype=yaml.ansible, the keyword "children" should be highlighted in the same way that "hosts" is highlighted.

syntax/ansible.vim

@@ -88,7 +88,7 @@ if exists("g:ansible_extra_keywords_highlight")
               \ debugger always_run check_mode diff no_log args tags force_handlers
               \ vars vars_files vars_prompt delegate_facts delegate_to
               \ any_errors_fatal ignore_errors ignore_unreachable max_fail_percentage
-              \ connection hosts port remote_user module_defaults
+              \ connection children hosts port remote_user module_defaults
               \ environment fact_path gather_facts gather_subset gather_timeout
               \ async poll throttle timeout order run_once serial strategy
               \ containedin='.s:yamlKey.' contained'