ENH: Fix return value type annotation in GP error analysis script func

Fix return value type annotation in GP error analysis script function.

Fixes:
```
scripts/dwi_gp_estimation_error_analysis.py:78: error:
 Incompatible return value type (got "ndarray[Any, Any]",
 expected "dict[int, list[tuple[ndarray[Any, Any], ndarray[Any, Any], ndarray[Any, Any], ndarray[Any, Any]]]]")  [return-value]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:111

Author: jhlegarreta

scripts/dwi_gp_estimation_error_analysis.py

@@ -49,7 +49,7 @@ def cross_validate(
     cv: int,
     n_repeats: int,
     gpr: DiffusionGPR,
-) -> dict[int, list[tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray]]]:
+) -> np.ndarray:
     """
     Perform the experiment by estimating the dMRI signal using a Gaussian process model.
 
@@ -68,7 +68,7 @@ def cross_validate(
 
     Returns
     -------
-    :obj:`dict`
+    :obj:`~numpy.ndarray`
         Data for the predicted signal and its error.
 
     """