Update parameter name for clarity

Updated the `HasOne` parameter name to be consistent with `HasForeignKey` so as not to cause confusion. Resolves dotnet#1626

samples/core/Modeling/FluentAPI/Relationships/OneToOne.cs

@@ -12,7 +12,7 @@ class MyContext : DbContext
         protected override void OnModelCreating(ModelBuilder modelBuilder)
         {
             modelBuilder.Entity<Blog>()
-                .HasOne(p => p.BlogImage)
+                .HasOne(b => b.BlogImage)
                 .WithOne(i => i.Blog)
                 .HasForeignKey<BlogImage>(b => b.BlogForeignKey);
         }