Use pages.markdown.extra in system.yaml (#1007)
Use pages.markdown.extra in system.yaml (#1007)

Markdown Extra is incorrectly specified as `markdown_extra`, whereas it should be:

```
  markdown:
    extra: false
```
joeyhenricks/a-broke-vegan,sylvestrelucia/grav,userfrosting/learn,notklaatu/grav,tomzx/blog.tomrochette.com,Vivalldi/grav,getgrav/grav,dimayakovlev/grav,tomzx/blog.tomrochette.com,JamesPan/gravbox,Sommerregen/grav,gigago/grav,Vivalldi/grav,sylvestrelucia/grav,JamesPan/gravbox,userfrosting/learn,dimayakovlev/grav,sylvestrelucia/grav,ccardinaux/grav-test,stell/grav,notklaatu/grav,getgrav/grav,JamesPan/gravbox,joeyhenricks/a-broke-vegan,userfrosting/learn,tomzx/blog.tomrochette.com,JamesPan/gravbox,tomzx/blog.tomrochette.com,sylvestrelucia/grav,webuxr/iamrs,userfrosting/learn,shortsofia/testesitegrav.github.io