Fix gen-doc task

gen-asciidoc: [exec] python3 ./scripts/gen-nodetool-docs.py [exec] python3 ./scripts/convert_yaml_to_adoc.py ../conf/cassandra.yaml ./modules/cassandra/pages/configuration/cass_yaml_file.adoc [exec] Traceback (most recent call last): [exec] File "/home/jlewandowski/dev/cassandra/trunk/doc/./scripts/convert_yaml_to_adoc.py", line 144, in <module> [exec] convert(yaml_file, dest_file) [exec] File "/home/jlewandowski/dev/cassandra/trunk/doc/./scripts/convert_yaml_to_adoc.py", line 61, in convert [exec] with open(dest_file, 'w') as outfile: [exec] FileNotFoundError: [Errno 2] No such file or directory: './modules/cassandra/pages/configuration/cass_yaml_file.adoc' [exec] make: *** [Makefile:26: gen-asciidoc] Error 1 [exec] Result: 2