docs: fix syntax highlighting for shell scripts. (#24103)

Switch all instances to `sh` but keep `bash` so that things won't break either way.

assets/scss/_syntax.scss

@@ -62,5 +62,15 @@
 .css .o + .nt,
 .css .nt + .nt { color: #999; }
 
-.language-bash::before { color: #009; content: "$ "; user-select: none; }
-.language-powershell::before { color: #009; content: "PM> "; user-select: none; }
+.language-bash::before,
+.language-sh::before {
+  color: #009;
+  content: "$ ";
+  user-select: none;
+}
+
+.language-powershell::before {
+  color: #009;
+  content: "PM> ";
+  user-select: none;
+}

docs/4.0/getting-started/download.md

@@ -52,7 +52,7 @@ Pull in Bootstrap's **source files** into nearly any project with some of the mo
 
 Install Bootstrap in your Node powered apps with [the npm package](https://www.npmjs.com/package/bootstrap):
 
-{% highlight bash %}
+{% highlight sh %}
 npm install bootstrap@{{ site.current_version }}
 {% endhighlight %}
 
@@ -73,7 +73,7 @@ gem 'bootstrap', '~> 4.0.0.beta'
 
 Alternatively, if you're not using Bundler, you can install the gem by running this command:
 
-{% highlight bash %}
+{% highlight sh %}
 gem install bootstrap -v 4.0.0.beta
 {% endhighlight %}
 
@@ -83,7 +83,7 @@ gem install bootstrap -v 4.0.0.beta
 
 You can also install and manage Bootstrap's Sass and JavaScript using [Composer](https://getcomposer.org):
 
-{% highlight bash %}
+{% highlight sh %}
 composer require twbs/bootstrap:{{ site.current_version }}
 {% endhighlight %}