docs: fix incorrect --next flag in install instructions (#2222)

The install script does not accept --next as documented in the Steam Deck
guide. The script actually supports:
- --v2 to install the v2 Go release candidate
- --version (or -V) to specify a release tag

Updated docs/posts/steamdeck_guide.md to document the correct flags that
match what './install --help' actually supports.

Fixes 89luca89/distrobox#2147

Co-authored-by: Alessio Biancalana <alessio@dottorblaster.it>

docs/posts/steamdeck_guide.md

@@ -7,11 +7,18 @@ Add the following line if it’s not already there:
 `export PATH=/home/deck/.local/bin:$PATH`
 
 **2 - Install and configure Distrobox**
-To install Distrobox in the defined $PATH, use one of the following commands
-depending on whether you need the latest version (`--next`) or not:
+To install Distrobox in the defined $PATH, use the following command:
 
 `curl -s https://raw.githubusercontent.com/89luca89/distrobox/main/install | sh -s -- --prefix $HOME/.local`
 
+This installs the v1 stable release by default. To install the v2 Go release candidate instead, add `--v2`:
+
+`curl -s https://raw.githubusercontent.com/89luca89/distrobox/main/install | sh -s -- --prefix $HOME/.local --v2`
+
+You can also specify a particular release tag with `--version` (or `-V`):
+
+`curl -s https://raw.githubusercontent.com/89luca89/distrobox/main/install | sh -s -- --prefix $HOME/.local --version 1.8.2.5`
+
 After installing, create the file `~/.distroboxrc` if it doesn't already exist.
 Open it with: