dataset id path should be identifier as per the schema

[odscjen]

Currently in the spreadsheet on sheet datasets cell B1 is id but it should be identifier

[duncandewhurst]

I've changed identifier to id in the schema in GFDRR/rdl-standard#181 because Flatten Tool requires identifiers to have consistent field names and we've used id everywhere but the top-level object. Therefore, there's no change required to the spreadsheet template.