Fix MSBuild condition

[martincostello]

Fix MSBuild property reference missing a $.

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[WeihanLi]

Maybe should align with the following condition

Suggested change

	<PackageReference Include="System.ValueTuple" Version="4.5.0" Condition="$(TargetFramework.StartsWith('net46')) or $(TargetFramework) != '.NETStandard'" />
	<PackageReference Include="System.Memory" Version="4.5.5" Condition="$(TargetFramework) != '.NETStandard' or $(TargetFrameworkIdentifier) == '.NETFramework'" />
	<PackageReference Include="System.ValueTuple" Version="4.5.0" Condition="'$(TargetFramework)' == 'netstandard2.0' or $(TargetFrameworkIdentifier) == '.NETFramework" />
	<PackageReference Include="System.Memory" Version="4.5.5" Condition="'$(TargetFramework)' == 'netstandard2.0' or $(TargetFrameworkIdentifier) == '.NETFramework" />

[martincostello]

I can change that if the maintainers ask, but that's a completely different change to just fixing the existing broken condition.

[SimonCropp]

@martincostello thanks for this

@clairernovotny this probably justifies a release