Fixing a typo in the Quick Start page

[FranklinLiang]

Under the Add content section, a backslash was missing from the command to add a new page.

[jmooring]

All of these work with v0.112.0 or later, which is why the quick start guide requires v0.112.0 or later.

hugo new content content/posts/foo.md
hugo new content posts/foo.md
hugo new content/posts/foo.md
hugo new posts/foo.md

We currently don't require the "content" subcommand; this provides backward compatibility. The "content" subcommand was added in v0.112.0.

[jmooring]

Thanks!