The PackageLicenseExpression was incorrectly using the BSD2 expression. This does not correcly match the projects license of BSD3 https://github.com/shouldly/shouldly?tab=License-1-ov-file

fixes #941

This effectively makes it clear that clause 3 applies to both the packaged nuget and the source code consistently

3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.