bindgen/default: Improve formatting of the readme

Place package names and versions between backticks to format them as
monospaced inline code making it easier to spot and copy, and wrap links
between angle brackets to satisfy various Markdown linters.

Finally, apply shell formatting to the `mdmerge` oneliner script.

crates/libs/bindgen/default/readme.md

@@ -1,23 +1,28 @@
 These `.winmd` files provide the default metadata for the Windows API. This is used to
 generate the `windows` and `windows-sys` crates. To view the metadata, use a tool
-like [ILSpy](https://github.com/icsharpcode/ILSpy). 
+like [ILSpy](https://github.com/icsharpcode/ILSpy).
 
-## Windows.Win32.winmd
-- Source: https://www.nuget.org/packages/Microsoft.Windows.SDK.Win32Metadata/
-- Version: 58.0.18
+## `Windows.Win32.winmd`
 
-## Windows.Wdk.winmd
-- Source: https://www.nuget.org/packages/Microsoft.Windows.WDK.Win32Metadata/
-- Version: 0.10.7
+- Source: <https://www.nuget.org/packages/Microsoft.Windows.SDK.Win32Metadata/>
+- Version: `58.0.18`
 
-## Windows.winmd
-- Source: https://www.nuget.org/packages/Microsoft.Windows.SDK.Contracts
-- Version: 10.0.22621.2428
+## `Windows.Wdk.winmd`
+
+- Source: <https://www.nuget.org/packages/Microsoft.Windows.WDK.Win32Metadata/>
+- Version: `0.10.7`
+
+## `Windows.winmd`
+
+- Source: <https://www.nuget.org/packages/Microsoft.Windows.SDK.Contracts>
+- Version: `10.0.22621.2428`
 
 The `Windows.winmd` file was created by merging the .winmd files from the last nuget package as follows:
 
-```
+```sh
 mdmerge -o out -i . -n 1
 ```
 
+---
+
 As with everything else in this repo, the `.winmd` files in this folder are licensed via MIT or Apache-2.0.