Ignore malformed YAML spec files in Prettier

The spec/ogcapi-connectedsystems-2.bundled.oas31.yaml file has
syntax errors and cannot be parsed by Prettier. Adding spec/**/*.yaml
to .prettierignore to prevent format:check failures.

Author: Sam-Bolling

.prettierignore

@@ -8,3 +8,5 @@ app/node_modules
 coverage
 temp-coverage-map.json
 jest-results.json
+
+spec/**/*.yaml