Update strings.xml

Author: tibbi

app/src/main/res/values-fa/strings.xml

@@ -14,7 +14,7 @@
     <string name="disable_shuffle">غیر فعال کردن پخش تصادفی</string>
     <string name="shuffle_enabled">پخش تصادفی فعال شد</string>
     <string name="shuffle_disabled">پخش تصادفی غیر فعال شد</string>
-    <string name="sleep_timer">Sleep timer</string>
+    <string name="sleep_timer">تایمر خواب</string>
     <string name="volume">حجم صدا</string>
     <string name="playback_speed">Playback speed</string>
     <string name="loading_files">Loading files…</string>
@@ -93,4 +93,4 @@
        Haven't found some strings? There's more at
        https://github.com/SimpleMobileTools/Simple-Commons/tree/master/commons/src/main/res
    -->
-</resources>
+</resources>