CLIENT-SPECIFICATION: Use main for asset link

[MasterOdin]

PR updates the client spec doc so that the link to the asset.zip file in the raw tldr-pages.github.io URL uses the main branch, instead of the master branch. This probably should have been done in a66723d as part of #5110.

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[CLAassistant]

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