Fix unused-result warnings in the contains and parse_error examples (#5788)

contains(json_pointer) is marked JSON_HEDLEY_WARN_UNUSED_RESULT since #5477,
and parse() has been for longer. The contains example ignored the result in
two try blocks waiting for a parse_error that contains() never throws, so
they printed nothing; print the result for those pointers instead.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/examples/contains__json_pointer.output

@@ -5,3 +5,5 @@ true
 false
 false
 false
+false
+false