Typos and confusing docs in the "Go Generated Code Guide (Open)"

[abenn135]

There's a few typos and mis-documentations in the Go generated code guide:

• "Singular Explicit Presence Scalar Fields" and "Singular Implicit Presence Scalar Fields" have the same field definition, yet they claim to generate different structs, one with a BirthYear int32 and one with a BirthYear *int32.
• The docs reference FirstActiveYear but no field of that name exists at HEAD. It was removed in Documentation update #266. cc @Logofile
• Message "Concer" is misspelled.

Thanks for taking a look!