Fix System.Xml reference case

Author: NicolasPetton

Xero.Api/Xero.Api.csproj

@@ -42,7 +42,7 @@
     <Reference Include="System.Data.Linq" />
     <Reference Include="System.Net.Http, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL" />
     <Reference Include="System.Runtime.Serialization" />
-    <Reference Include="System.XML" />
+    <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Common\IHasAttachment.cs" />