fix: correct two typos in schema and documentation descriptions

[avalyset]

Corrects two spelling mistakes in description text. Each appears twice: once in the schema
and once in the corresponding documentation page.

File	Line	Was	Now
schemas/security.json	426	purcahses	purchases
documentation/properties/gross_purchase_eur.md	11	purcahses	purchases
extensions/schemas/loan.json	288	arrangment	arrangement
extensions/documentation/properties/mod_date.md	11	arrangment	arrangement

The schema occurrences are the description of gross_purchase_eur (security) and mod_date
(loan extension).

Only description text is changed. No property name, enum value, type, or any other schema
key is touched, so the wire contract is unchanged. Both misspellings were checked against
every JSON key and every enum value in schemas/ and extensions/schemas/ first, and appear
in neither.

Both edited JSON files still parse. pytest --ignore=tests/test_extensions.py gives
231 passed, 2 skipped; tests/test_extensions.py was not run because iso3166 is not
available in this environment.