Revert "Update snap installation instructions"

This reverts commit 5fd7bf7e61.

Apparently it is not just no longer necessary to use classic
confinement, but actually impossible, i.e. snap emits an error that the
zola snap is not compatible with `--classic`.

docs/content/documentation/getting-started/installation.md

@@ -27,7 +27,7 @@ $ yay -S zola-bin
 Zola is available on snapcraft:
 
 ```bash
-$ snap install --edge --classic zola
+$ snap install --edge zola
 ```
 
 ## Windows