update usage.rst
This fixes the comment which is currently incorrect. pisa_status.err is a count of the errors and when 0 Python interprets this as False, and when greater than 0 Python interprets it as True, so I swapped the True and False around in the comment to reflect this.
@@ -25,7 +25,7 @@ Using with Python standalone
# close output file
result_file.close() # close output file
- # return True on success and False on errors
+ # return False on success and True on errors
return pisa_status.err
# Main program
