Update snap install instructions with beta channel

docs/INSTALL.md

@@ -1,20 +1,16 @@
 # Installation
 
 * [macOS with Homebrew](#macos-with-homebrew)
-* [Linux with Snap](#linux-with-snap)
 * [One-line binary install](#one-line-binary-install)
 * [Pre-built packages and binaries](#pre-built-packages-and-binaries)
 * [From source](#from-source)
+* [Linux with Snap (beta)](#linux-with-snap-beta)
 * [Upgrading](#upgrading)
 
 ## macOS with Homebrew
 
     brew install twpayne/taps/chezmoi
 
-## Linux with Snap
-
-    snap install chezmoi --classic
-
 ## One-line binary install
 
     curl -sfL https://git.io/chezmoi | sh
@@ -35,6 +31,10 @@
 
     cd $(mktemp -d) && go get -u github.com/twpayne/chezmoi
 
+## Linux with Snap (beta)
+
+    snap install chezmoi --channel=beta --classic
+
 ## Upgrading
 
 Once chezmoi is installed, you can upgrade it to the latest release with: