docs: correct typo in signin.mdx (#12939)

In additionally -> Additionally

Co-authored-by: Thang Vu <[email protected]>

Author: nougat-sw

docs/pages/guides/pages/signin.mdx

@@ -5,7 +5,7 @@ import { Screenshot } from "@/components/Screenshot"
 
 To add a custom sign-in page, you'll need to define the path to your page in the [`pages`](/reference/core/types#pagesoptions) object in your Auth.js configuration. Make sure a route / page actually exists at the path you're defining here!
 
-In additionally, we'll have to export a map of `provider.id` and `provider.name` to easily consume in our custom page if we want to dynamically render the correct buttons, based on what we've defined in our `auth.ts` configuration. Because you can pass your providers to the `providers` array as both a function, or the result of calling that function, this example `providerMap` handles both cases.
+Additionally, we'll have to export a map of `provider.id` and `provider.name` to easily consume in our custom page if we want to dynamically render the correct buttons, based on what we've defined in our `auth.ts` configuration. Because you can pass your providers to the `providers` array as both a function, or the result of calling that function, this example `providerMap` handles both cases.
 
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