Updated license package expression

Author: hiranya911

FirebaseAdmin/FirebaseAdmin/FirebaseAdmin.csproj

@@ -17,7 +17,7 @@
     <PackageTags>firebase;Google;Cloud</PackageTags>
     <Copyright>Copyright 2018 Google LLC</Copyright>
     <Authors>Google Inc.</Authors>
-    <PackageLicenseUrl>https://www.apache.org/licenses/LICENSE-2.0</PackageLicenseUrl>
+    <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/Firebase/firebase-admin-dotnet</PackageProjectUrl>
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/Firebase/firebase-admin-dotnet</RepositoryUrl>