docs: the temporal foreign key samples do not compile (HasTemporalForeignKey needs both type arguments) #76

Problem

docs/postgresql-constraints.md (lines 104 and 126) shows

modelBuilder.Entity<SubscriptionAddOn>(b =>
{
    b.HasTemporalForeignKey<Subscription>( … );
});

which fails with CS1061: 'EntityTypeBuilder<SubscriptionAddOn>' does not contain a definition for 'HasTemporalForeignKey' (SDK 10.0.402, EFCore.ComplexIndexes.PostgreSQL 5.5.0 candidate). The method is declared in a C# 14 extension<TEntity>(EntityTypeBuilder<TEntity>) block with its own TPrincipal, so an explicit type-argument list must name both: HasTemporalForeignKey<SubscriptionAddOn, Subscription>(…). The repository's own tests (NpgsqlNameCollisionTests) use the two-argument form. DocumentationApiTests checks that the method name exists, so it cannot see this.

Fix

Use the two-argument form in both samples.

Found by the 5.5.0 consumer probe, which compiled about 45 snippets from the READMEs and docs; this was the only one that did not compile.