Minor correction in Rust framework guide

[AshitaGK]

Title: Axum repeated in other Rust frameworks for installation.

Source: https://fly.io/docs/rust/frameworks/actix-web/

Issue: Minor correction but it seems to be repeated in every other Rust framework. The line 'If you don’t already have an existing Axum application' needs to be modified according to the rust framework you want to install.