Change error message to "...a palette chunk"

src/ImageSharp/Formats/Png/PngThrowHelper.cs

@@ -22,7 +22,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         public static void ThrowNoData() => throw new InvalidImageContentException("PNG Image does not contain a data chunk");
 
         [MethodImpl(InliningOptions.ColdPath)]
-        public static void ThrowMissingPalette() => throw new InvalidImageContentException("PNG Image does not contain palette chunk");
+        public static void ThrowMissingPalette() => throw new InvalidImageContentException("PNG Image does not contain a palette chunk");
 
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowInvalidChunkType() => throw new InvalidImageContentException("Invalid PNG data.");

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -278,7 +278,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
                     image.DebugSave(provider);
                 });
             Assert.NotNull(ex);
-            Assert.Contains("PNG Image does not contain palette chunk", ex.Message);
+            Assert.Contains("PNG Image does not contain a palette chunk", ex.Message);
         }
 
         [Theory]