ruff: Fix ICN002 geopandas import alias

[seisman]

ruff v0.15.3 starts to report the following error:

ruff check pygmt doc/conf.py examples
ICN002 `geopandas` should not be imported as `gpd`
  --> examples/gallery/lines/linestrings.py:15:1
   |
14 | # %%
15 | import geopandas as gpd
   | ^^^^^^^^^^^^^^^^^^^^^^^
16 | import pygmt
   |

ICN002 `geopandas` should not be imported as `gpd`

which was introduded in astral-sh/ruff#21373.

It seems the GeoPandas site also uses import geopandas rather than import geopandas as gpd (xref: https://geopandas.org/en/stable/getting_started/introduction.html).

This PR fixes the issue.