fix(billing): Update Spend Notifications link (#71137)

Update "Learn More" link that will point to updated documentation in
https://github.com/getsentry/sentry-docs/pull/10076.

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

static/app/views/settings/account/notifications/fields2.tsx

@@ -226,7 +226,13 @@ export const SPEND_FIELDS = [
     help: tct(
       'Receive notifications when your spend crosses predefined or custom thresholds. [learnMore:Learn more]',
       {
-        learnMore: <ExternalLink href={'#'} />, // TODO(isabella): replace with proper link
+        learnMore: (
+          <ExternalLink
+            href={
+              'https://docs.sentry.io/product/alerts/notifications/#spend-notifications'
+            }
+          />
+        ),
       }
     ),
     choices: [