Update docs to fix markdown and square bracket use
What does this MR do?
In 3 docs, the use of square brackets instead of parentheses were causing some kramdown warnings, and likely not rendering as intended. This cleans up those 3 docs.
$ bundle exec nanoc
Loading site… done
kramdown warning(s) for <Nanoc::CompilationItemRepView item.identifier=/docs/executors/kubernetes.md name=default>
No link definition for link ID 'advanced' found on line 84
No link definition for link ID 'default: 3' found on line 86
No link definition for link ID 'default: 180' found on line 87
No link definition for link ID 'string' found on line 1
No link definition for link ID 'string' found on line 1
kramdown warning(s) for <Nanoc::CompilationItemRepView item.identifier=/docs/configuration/advanced-configuration.md name=default>
No link definition for link ID 'advanced' found on line 1
kramdown warning(s) for <Nanoc::CompilationItemRepView item.identifier=/docs/commands/README.md name=default>
No link definition for link ID 'prompt' found on line 570
No link definition for link ID 'prompt' found on line 570