[DOC] plot type heading consistency (matplotlib#28254)

Just removed the colon from the Gridded Data heading b/c we don't colons after any of the others

---------

Co-authored-by: Tim Hoffmann <[email protected]>

Author: story645

galleries/plot_types/arrays/README.rst

@@ -1,7 +1,7 @@
 .. _array_plots:
 
-Gridded data:
--------------
+Gridded data
+------------
 
 Plots of arrays and images :math:`Z_{i, j}` and fields :math:`U_{i, j}, V_{i, j}`
 on `regular grids <https://en.wikipedia.org/wiki/Regular_grid>`_ and