Misaligned help text with positional arguments and numpydoc

When using Numpydoc docstring style with multiple positional arguments the help text gets misaligned

```
import fire

def run(executable, argument):
"""Run.

Parameters
----------
executable :
Executable to run
argument :
Positional argument to the executable

"""
print("hello")

if __name__ == "__main__":
fire.Fire(run)
```

When run with `python test.py --help` this code produces

```
INFO: Showing help with the command 'test.py -- --help'.

NAME
test.py - Run.

SYNOPSIS
test.py EXECUTABLE ARGUMENT

DESCRIPTION
Run.

POSITIONAL ARGUMENTS
EXECUTABLE
Executable to run

Positional argument to the executable
ARGUMENT

NOTES
You can also use flags syntax for POSITIONAL ARGUMENTS
```

where the help text to `argument` gets added to `executable`.