nixos/doc: fix a typo in option-declarations.section.md (#304733)

## Extensible Option Types {#sec-option-declarations-eot}

Extensible option types is a feature that allow to extend certain types

Extensible option types is a feature that allows to extend certain types

declaration through multiple module files. This feature only work with a

restricted set of types, namely `enum` and `submodules` and any composed

forms of them.