Deploy dev docs - fix sed command (bevyengine#4221)

# Objective

- bevyengine#3535 introduced deploying docs with an error in sed command

## Solution

- fix sed command
- make the command cross platform

.github/workflows/docs.yml

@@ -28,7 +28,7 @@ jobs:
       #   - Adds a meta tag that forces Google not to index any page on the site.
       - name: Pre-docs-build
         run: |
-          sed -i "s/icon.png/icon-docs-dev.png" src/lib.rs
+          sed -i.bak "s/icon.png/icon-docs-dev.png/" src/lib.rs
           echo "<meta name=\"robots\" content=\"noindex\">" > header.html
 
       - name: Build docs