docs: Fix tee command for debian instructions

`tee -a` makes no sense here, we don't want to append, overwriting is just fine

Author: francislavoie

src/docs/markdown/install.md

@@ -46,15 +46,15 @@ Stable releases:
 
 <pre><code class="cmd"><span class="bash">sudo apt install -y debian-keyring debian-archive-keyring apt-transport-https</span>
 <span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/gpg.key' | sudo apt-key add -</span>
-<span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | sudo tee -a /etc/apt/sources.list.d/caddy-stable.list</span>
+<span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | sudo tee /etc/apt/sources.list.d/caddy-stable.list</span>
 <span class="bash">sudo apt update</span>
 <span class="bash">sudo apt install caddy</span></code></pre>
 
 Testing releases (includes betas and release candidates):
 
 <pre><code class="cmd"><span class="bash">sudo apt install -y debian-keyring debian-archive-keyring apt-transport-https</span>
 <span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/testing/gpg.key' | sudo apt-key add -</span>
-<span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/testing/debian.deb.txt' | sudo tee -a /etc/apt/sources.list.d/caddy-testing.list</span>
+<span class="bash">curl -1sLf 'https://dl.cloudsmith.io/public/caddy/testing/debian.deb.txt' | sudo tee /etc/apt/sources.list.d/caddy-testing.list</span>
 <span class="bash">sudo apt update</span>
 <span class="bash">sudo apt install caddy</span></code></pre>