Use more concise home feature title (nushell#1970)

Use `Cross-platform` instead of `Pipelines to control any OS`.

It's not necessarily clear what pipelines refers to. Pipelines may refer to one of many use cases. Pipelines may be a misleading ambiguity to command piping (shell terminology).

The previous title lead to a line break. Making it more concise to fit on one line is more consistent with the other two feature titles - leading to a more consistent and balanced layout too. (Body starts on the same vertical position now.)

Author: Kissaki

README.md

@@ -6,7 +6,7 @@ tagline: A new type of shell
 actionText: Get Started →
 actionLink: /book/
 features:
-  - title: Pipelines to control any OS
+  - title: Cross-platform
     details: Nu works on Linux, macOS, BSD, and Windows. Learn it once, then use it anywhere.
   - title: Everything is data
     details: Nu pipelines use structured data so you can safely select, filter, and sort the same way every time. Stop parsing strings and start solving problems.