Update the title capitalization style (#2624)

Task/Issue URL:
https://app.asana.com/0/1199230911884351/1206924693423599/f

**Description**:
"Your subscription is being activated" should be in sentence case.

**Steps to test this PR**:
Getting the "Your subscription is being activated" state during
subscription purchase may be hard to replicate as it requires BE
failure.
To force trigger the state, please hardcode for the `statePublisher` in
`PreferencesSubscriptionModel` to always return
`PreferencesSubscriptionState.subscriptionPendingActivation`.



---
###### Internal references:
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[Software Engineering
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[Technical Design
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[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

LocalPackages/SubscriptionUI/Sources/SubscriptionUI/UserText.swift

@@ -75,7 +75,7 @@ enum UserText {
     static let haveSubscriptionButton = NSLocalizedString("subscription.preferences.i.have.a.subscription.button", value: "I Have a Subscription", comment: "Button enabling user to activate a subscription user bought earlier or on another device")
 
     // MARK: Preferences when subscription activation is pending
-    static let preferencesSubscriptionPendingHeader = NSLocalizedString("subscription.preferences.subscription.pending.header", value: "Your Subscription is Being Activated", comment: "Header for the subscription preferences pane when the subscription activation is pending")
+    static let preferencesSubscriptionPendingHeader = NSLocalizedString("subscription.preferences.subscription.pending.header", value: "Your subscription is being activated", comment: "Header for the subscription preferences pane when the subscription activation is pending")
     static let preferencesSubscriptionPendingCaption = NSLocalizedString("subscription.preferences.subscription.pending.caption", value: "This is taking longer than usual, please check back later.", comment: "Caption for the subscription preferences pane when the subscription activation is pending")
 
     // MARK: Preferences when subscription is expired