Docs: Fix markdown typos (#26386)

fixed some markdown code format in the debian install docs.

docs/sources/installation/debian.md

@@ -39,6 +39,7 @@ If you install from the APT repository, then Grafana is automatically updated ev
 sudo apt-get install -y apt-transport-https
 sudo apt-get install -y software-properties-common wget
 wget -q -O - https://packages.grafana.com/gpg.key | sudo apt-key add -
+```
 
 Add this repository for stable releases:
 
@@ -53,7 +54,7 @@ echo "deb https://packages.grafana.com/enterprise/deb beta main" | sudo tee -a /
 
 After you add the repository:
 
-```
+```bash
 sudo apt-get update
 sudo apt-get install grafana-enterprise
 ```
@@ -79,7 +80,7 @@ echo "deb https://packages.grafana.com/oss/deb beta main" | sudo tee -a /etc/apt
 
 After you add the repository:
 
-```
+```bash
 sudo apt-get update
 sudo apt-get install grafana
 ```