Update secure-storage.md (#1859)

Fix incorrect logic when removing a key from legacy storage on Android. The old logic cleared all preferences instead of removing the singular key as intended. The new logic now only removes the desired key.

Author: ahoffmandevelopment

docs/migration/secure-storage.md

@@ -67,7 +67,7 @@ public class LegacySecureStorage
         bool result = false;
 
 #if ANDROID
-        Preferences.Clear(Alias);
+        Preferences.Remove(key, Alias);
         result = true;
 #elif IOS
         KeyChain keyChain = new KeyChain();