fix image warnings

vlc-ios/Images.xcassets/MenuNavigation/Tab bar/Audio.imageset/Contents.json

@@ -7,12 +7,12 @@
     },
     {
       "idiom" : "universal",
-      "filename" : "Audio@2x.png",
+      "filename" : "audio@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "Audio@3x.png",
+      "filename" : "audio@3x.png",
       "scale" : "3x"
     }
   ],

vlc-ios/Images.xcassets/MenuNavigation/Tab bar/Network.imageset/Contents.json

@@ -7,12 +7,12 @@
     },
     {
       "idiom" : "universal",
-      "filename" : "Network@2x.png",
+      "filename" : "network@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "Network@3x.png",
+      "filename" : "network@3x.png",
       "scale" : "3x"
     }
   ],

vlc-ios/Images.xcassets/MenuNavigation/Tab bar/Settings.imageset/Contents.json

@@ -7,12 +7,12 @@
     },
     {
       "idiom" : "universal",
-      "filename" : "Settings@2x.png",
+      "filename" : "settings@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "Settings@3x.png",
+      "filename" : "settings@3x.png",
       "scale" : "3x"
     }
   ],

vlc-ios/Images.xcassets/MenuNavigation/Tab bar/Video.imageset/Contents.json

@@ -7,12 +7,12 @@
     },
     {
       "idiom" : "universal",
-      "filename" : "Video@2x.png",
+      "filename" : "video@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "Video@3x.png",
+      "filename" : "video@3x.png",
       "scale" : "3x"
     }
   ],