Minor updates to csproj files.
MODIFY
src/Chapter20/Chapter20.csproj
src/Chapter20/Chapter20.csproj
<PropertyGroup>
<ChapterNumber>20</ChapterNumber>
<WarningLevel>2</WarningLevel>
- <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
</PropertyGroup>
<Import Project="..\Chapter.props" />
<ItemGroup>

Minor updates to csproj files.