Fix arg type in docstring for (*Po).Parse

[m90]

Is this a fix, improvement or something else?

Documentation fix.

What does this change implement/fix?

The Godoc string for (*Po).Parse currently does not match the function's signature.

I have ...

• [x] answered the 2 questions above,
• [ ] discussed this change in an issue,
• [ ] included tests to cover this changes.

[leonelquinteros]

Good catch, thanks for the fix, it's really appreciated!