docs: standardize 'color' spelling in alert component (#3148)

docs/content/ui/components/alert.mdx

@@ -307,5 +307,5 @@ You're not limited to the 4 default alert colors. You can use any [base color](/
 - The live region has to exist before the message arrives. Render the container on page load and put the text into it, rather than inserting the whole alert at the moment of the event.
 - Label the dismiss button with `aria-label="Close"`. `btn-close` has no text of its own.
 - The icon is decoration, so the alert must still read correctly with the icon ignored. Keep the meaning in the heading and the text.
-- Colour alone does not say "error". A red alert and a green one must differ in their words too.
+- Color alone does not say "error". A red alert and a green one must differ in their words too.
 - After dismissing an alert, move focus somewhere sensible. Focus left on a removed element falls back to the top of the page.