README.md: Fix missing period

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

README.md

@@ -138,7 +138,7 @@ Android's `update_engine` verifies OTA signatures against certificates contained
 * `REBOOT` (**automatically granted by system app permissions**)
   * Needed to reboot the device when the user explicitly presses the reboot button in Custota's notification after an update is installed.
 * `RECEIVE_BOOT_COMPLETED` (**automatically granted at install time**)
-  * Needed to schedule periodic update checks
+  * Needed to schedule periodic update checks.
 * `WAKE_LOCK` (**automatically granted at install time**)
   * Needed to keep the CPU awake while an update is being installed.