Include ":snippet_id" in "Update snippet" and "Delete snippet" example request URL

What does this MR do?

This MR updates the Update Snippet and Delete snippet examples to include the ":snippet_id" parameter in the URL. Existing example leaves out the ":snippet_id" which would result in a 404 if following the example. The ID is a "required" query parameter.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

No risks involved. Simply updates the example URL parameter in the documentation.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team