Fix nuspec PackageLicenseUrl

[Greg-Smulko]

Fix nuspec PackageLicenseUrl - it seems to be forgotten when migrating from gsscoder to commandlineparser.

Fixes #429.

[Greg-Smulko]

Fixed already in #549, as reported in #545, so closing it.
A bit sad that this PR stayed open for almost a year instead of just being merged, but that's life...

[moh-hassan]

@Greg-Smulko
The license is fixed by using license file , not URL, because within one year NUGET changes the license configuration more than one time and finally documented here.

[Greg-Smulko]

@moh-hassan
In the link that you provided, there is still "SPDX identifier or expression" as a method of documenting the license. Also, see in https://docs.microsoft.com/en-us/dotnet/core/tools/csproj#packagelicensefile that:

PackageLicenseFile
Path to a license file within the package if you are using a license that hasn’t been assigned an SPDX identifier, or it is a custom license (Otherwise PackageLicenseExpression is preferred)

That's why I still think that the expression is the preferred way for this repo - I even opened a PR for this a year ago, see #433. I rebased it on top of master a moment ago, so if you want to merge it, feel free. Otherwise, please just close it.