docs: fix broken demo link, typo and warning callout syntax in README

[xiakong130]

What changed

Three fixes in README.md:

1. Fixed broken git clone URL: garrone/gitlanding-demo -> garronej/gitlanding-demo (missing j in username, results in 404)
2. Fixed grammar: "components needs be inside" -> "components need to be inside"
3. Fixed GitHub alert syntax: [!WARNING]: ... -> multi-line [!WARNING] format (the inline-colon form does not render as a warning callout)

Why

• The demo clone command was broken — users following the Quick start guide would hit a 404
• The grammar error was in the usage instructions
• The WARNING callout was not rendering correctly on GitHub (compare with the NOTE callout below it, which uses the correct multi-line format)

How I checked

• Verified garronej/gitlanding-demo exists (the correct URL is also used later in the README at the Contributing section, line 56)
• Confirmed the diff only touches the three intended lines