[android_alarm_manager] fix AndroidManifest.xml for android lint issue "XML tag has empty body" (#3288)

diff --git a/packages/android_alarm_manager/README.md b/packages/android_alarm_manager/README.md
index e7c7f6e..9d245e5 100644
--- a/packages/android_alarm_manager/README.md
+++ b/packages/android_alarm_manager/README.md
@@ -36,7 +36,7 @@
     android:name="io.flutter.plugins.androidalarmmanager.RebootBroadcastReceiver"
     android:enabled="false">
     <intent-filter>
-        <action android:name="android.intent.action.BOOT_COMPLETED"></action>
+        <action android:name="android.intent.action.BOOT_COMPLETED"/>
     </intent-filter>
 </receiver>
 
diff --git a/packages/android_alarm_manager/example/android/app/src/main/AndroidManifest.xml b/packages/android_alarm_manager/example/android/app/src/main/AndroidManifest.xml
index 356c10f..2a9dc33 100644
--- a/packages/android_alarm_manager/example/android/app/src/main/AndroidManifest.xml
+++ b/packages/android_alarm_manager/example/android/app/src/main/AndroidManifest.xml
@@ -41,7 +41,7 @@
             android:name="io.flutter.plugins.androidalarmmanager.RebootBroadcastReceiver"
             android:enabled="false">
             <intent-filter>
-                <action android:name="android.intent.action.BOOT_COMPLETED"></action>
+                <action android:name="android.intent.action.BOOT_COMPLETED"/>
             </intent-filter>
         </receiver>
         <meta-data