docs: update long-description

[SkyZeroZx]

What is the current behavior?

What is the new behavior?

Avoid using absolute links in angular.dev since this is an internal route.

[gemini-code-assist]

Code Review

This pull request updates the documentation for the ng update command. The changes involve updating an example version from 10 to 21 and converting an absolute URL for the Angular Update Guide to a relative path. While the link change is a good improvement, the hardcoded version number in the example could quickly become outdated. I've provided a suggestion to rephrase the example to make it more robust and clearer for users.

[gemini-code-assist]

While updating the version number makes the example more current, hardcoding a specific version like 21 can quickly become outdated and potentially confuse users if it doesn't align with the actual latest version. To make the documentation more timeless, consider rephrasing to make it clearer that this is an example of how to use the <major_version> placeholder mentioned above.

Suggested change

	For example, use the following command to take the latest 21.x.x version and use that to update.
	For example, to update to major version 21, you would use:

[SkyZeroZx]

Try just keeping it updated by updating the number; I'm not sure if we should change it.