Spec: Fix reserved field ID limit

[dannycjones]

According to the Java implementation, this requirement is inclusive or "greater than or equal" to this value.

iceberg/core/src/main/java/org/apache/iceberg/StatsUtil.java

Lines 54 to 58 in 3d4c6e0

	public static int statsFieldIdForField(int fieldId) {
	return fieldId >= RESERVED_FIELD_IDS_START
	? statsFieldIdForReservedField(fieldId)
	: statsFieldIdForDataField(fieldId);
	}

I am proposing to update the spec to clarify this. This appears to change the spec itself though, so I'm open to looking instead at changing this in the Java implementation to accommodate the difference with the spec.