docs: fix 4 typos in comments and messages

[Avicennasis]

Summary

Fix 4 small typos/copy-paste errors:

• neo4j/src/main/scala/magnolify/neo4j/ValueType.scala: @implicitNotFound("Cannot derive AvroField for sealed trait") -> ... ValueField .... Copy-paste from the avro module; every other module names its own typeclass here (ProtobufField, TableRowField, ExampleField, EntityField, BigtableField), so the neo4j derivation currently reports the wrong type in the compile error.
• parquet/src/main/scala/magnolify/parquet/Schema.scala: stray double backtick in the InvalidRecordException message - reader schema $reader``" ->reader schema `$reader`"`.
• bigquery/src/main/scala/magnolify/bigquery/TimestampConverter.scala: the doc link under the // TIMESTAMP heading pointed at data-types#time_type; changed to data-types#timestamp_type. (The #time_type link under the // TIME heading is correct and was left alone.)
• parquet/src/main/scala/magnolify/parquet/logical/package.scala: // TIME (millis i32, micros i64, nanos, i64) -> nanos i64 (stray comma).

Please note one change touches a string literal: the InvalidRecordException message in Schema.scala. No test in the repo asserts on that text (grepped), and the change only removes a duplicated backtick, but flagging it so it gets a second look. Everything else is comments/annotations.

No functional changes and no identifiers renamed.