Fix .NET 5.0 C# TryParse signature. (dotnet#5350)

In `TryParse`, `string value` was correctly updated to `string? value`, but `out bool result` was incorrectly updated to `out bool? result`.

Author: ILMTitan

xml/System/Boolean.xml

@@ -1993,7 +1993,7 @@ This field is equal to the string "True".
       <MemberSignature Language="VB.NET" Value="Public Shared Function TryParse (value As String, ByRef result As Boolean) As Boolean" />
       <MemberSignature Language="C++ CLI" Value="public:&#xA; static bool TryParse(System::String ^ value, [Runtime::InteropServices::Out] bool % result);" />
       <MemberSignature Language="F#" Value="static member TryParse : string * bool -&gt; bool" Usage="System.bool.TryParse (value, result)" />
-      <MemberSignature Language="C#" Value="public static bool TryParse (string? value, out bool? result);" FrameworkAlternate="net-5.0;netcore-3.0;netcore-3.1" />
+      <MemberSignature Language="C#" Value="public static bool TryParse (string? value, out bool result);" FrameworkAlternate="net-5.0;netcore-3.0;netcore-3.1" />
       <MemberType>Method</MemberType>
       <AssemblyInfo>
         <AssemblyName>System.Runtime</AssemblyName>