NuGet packages named "preiew" instead of "preview"

[rmunn]

There appears to be a typo in the build process: the version numbers listed at https://www.nuget.org/packages/EntityFrameworkCore.Projectables and https://www.nuget.org/packages/EntityFrameworkCore.Projectables.Abstractions are showing as "4.0.0-preiew.2" instead of "4.0.0-preview.2" (and same for 4.0.0-preview.1).

[ZvonimirMatic]

Created a PR to fix this.

#108

@rmunn @koenbeuk