Correct spelling in installation guide reference (#66484)

One-word 'checkout' is a noun or adjective that refers to the act of
taking items out of a store after paying for them. 2. Two-words 'check
out' is a verb that refers to request someone to look at something.
Corrected the spelling of "checkout" to "check out" in the documentation
to improve readability and accuracy. This change ensures proper usage of
the phrase when referring to the installation guide.

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Co-authored-by: Sam Ko <sam@vercel.com>

docs/index.mdx

@@ -32,7 +32,7 @@ On the left side of the screen, you'll find the docs navbar. The pages of the do
 
 On the right side of the screen, you'll see a table of contents that makes it easier to navigate between sections of a page. If you need to quickly find a page, you can use the search bar at the top, or the search shortcut (`Ctrl+K` or `Cmd+K`).
 
-To get started, checkout the [Installation](/docs/getting-started/installation) guide.
+To get started, check out the [Installation](/docs/getting-started/installation) guide.
 
 ## App Router vs Pages Router