Update Fonts ref

Author: JimBobSquarePants

src/ImageSharp.Drawing/ImageSharp.Drawing.csproj

@@ -20,7 +20,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="SixLabors.Fonts" Version="1.0.0-beta15.20" />
+    <PackageReference Include="SixLabors.Fonts" Version="1.0.0-beta15.21" />
     <PackageReference Include="SixLabors.ImageSharp" Version="1.0.4" />
   </ItemGroup>