[documentation] fix a typo

Author: zombieFox

README.md

@@ -53,7 +53,7 @@ Firefox does not allow the "homepage" to be replaces by an addon. However there
 - In Firefox preferences under `Home`, change `Homepage and new windows` to `Custom URLs...`
 - Then click `Use Current Page`.
 
-nightTab will now appear when the homepage.
+nightTab will now appear as the homepage.
 
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