Fix data type of amount field in writing_data_schema.md

[jdaviscooke]

While going through the getting started docs, we copied the schema provided and were getting the error

Provided schema is not valid.

How it could be resolved:

    Update schema descriptor to be a valid descriptor
    If this error should be ignored disable schema checks in goodtables.yml.

The full list of error messages:

    Table Schema error: Descriptor validation error: {'constraints': {'minimum': 0}, 'name': 'amount', 'format': 'default', 'type': 'numeric', 'description': 'Transaction value in Euros'} is not valid under any of the given schemas at "fields/3" in descriptor and at "properties/fields/items/anyOf" in profile

After reviewing the Table Schema documentation it seems that the correct type is number rather than numeric. This updates the documentation so that validation succeeds for the example.

[roll]

@jdaviscooke Thanks!