Fix typo `allow_override_time` -> `allow_time_override`.

[stefanozanella]

The mismatch between the exported option and the variable checked at runtime prevent the plugin to accept a custom time column.

[brupm]

@jordansissel seems simple enough to merge or close?

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[suyograo]

This has been fixed