[Bug]: Fix linux installation docs

[zbhavyai]

What happened?

Not really a bug, but a typo, in the documentation on installation of httpie desktop on a Linux machine. Its not really applicable to me as I started using the flatpak version, but thought better to get it updated.

Steps to reproduce?

1. Go to the URL https://httpie.io/docs/desktop/linux

2. Read the doc. The documentation talks about opening .exe file; it should say about AppImage as mentioned in the repo's README.

What did you expect to happen?

It should probably say something like this (copied from your README) or whatever marketing people decide -

---

Please follow the instructions below to install HTTPie for Desktop on Linux.

⚠️ The Linux package uses AppImage, and you need to install it using AppImageLauncher for the app to work:

1. Install AppImageLauncher (instructions)
2. Download HTTPie-*.AppImage (or HTTPie-*arm64.AppImage for ARM64 architecture) from the latest release page
3. Run $ AppImageLauncher HTTPie-*.AppImage

Platform

Linux

HTTPie Desktop version

2022.5.1

Relevant log output

No response

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