docs: specify lang for a codeblock (#4314)

Co-authored-by: bluwy <bjornlu.dev@gmail.com>

packages/integrations/preact/README.md

@@ -43,7 +43,7 @@ Because this command is new, it might not properly set things up. If that happen
 
 First, install the `@astrojs/preact` package using your package manager. If you're using npm or aren't sure, run this in the terminal:
 
-```
+```sh
 npm install @astrojs/preact
 ```
 

packages/integrations/turbolinks/README.md

@@ -39,7 +39,7 @@ If you run into any hiccups, [feel free to log an issue on our GitHub](https://g
 
 First, install the `@astrojs/turbolinks` integration like so:
 
-```
+```sh
 npm install @astrojs/turbolinks
 ```
 

packages/webapi/README.md

@@ -2,7 +2,7 @@
 
 **WebAPI** lets you use Web APIs in Node v12, v14, and v16.
 
-```shell
+```sh
 npm install @astrojs/webapi
 ```