fix: use relative import for formatters

[chriskd]

Quick fix for a relative import error, seemingly introduced in #33

Traceback (most recent call last):
  File "/Users/redacted/.cache/uv/archive-v0/THgpd63DeR44i3tg-_heo/bin/mcp_polygon", line 6, in <module>
    from mcp_polygon import run
  File "/Users/redacted/.cache/uv/archive-v0/THgpd63DeR44i3tg-_heo/lib/python3.13/site-packages/mcp_polygon/__init__.py", line 1, in <module>
    from .server import run
  File "/Users/redacted/.cache/uv/archive-v0/THgpd63DeR44i3tg-_heo/lib/python3.13/site-packages/mcp_polygon/server.py", line 7, in <module>
    from src.mcp_polygon.formatters import json_to_csv
ModuleNotFoundError: No module named 'src'

[joedursun]

Thanks for the quick fix, @chriskd!