DOC: Fix spelling mistakes detected by codespell

Fix spelling mistakes detected by `codespell`.

Fixes:
```
./README.rst:77: difussion ==> diffusion
./src/nifreeze/testing/simulations.py:70: pricipal ==> principal
./src/nifreeze/registration/ants.py:392: initalizing ==> initializing
spellcheck: exit 65 (0.17 seconds) /home/runner/work/nifreeze/nifreeze> codespell . pid=1810
  spellcheck: FAIL code 65 (0.27=setup[0.11]+cmd[0.17] seconds)
  evaluation failed :( (0.34 seconds)
Error: Process completed with exit code 65.
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12435719520/job/34722102640?pr=28#step:8:21

Author: jhlegarreta

README.rst

@@ -62,7 +62,7 @@ and positron-emission tomography (PET) data.
     the known directions and strengths of diffusion gradients*. J Magn Reson Imaging **24**:1188-1193.
 
 .. [4] Andersson et al. (2012) *A comprehensive Gaussian Process framework for correcting distortions
-    and movements in difussion images*. In: 20th SMRT & 21st ISMRM, Melbourne, Australia.
+    and movements in diffusion images*. In: 20th SMRT & 21st ISMRM, Melbourne, Australia.
 
 .. [5] Andersson & Sotiropoulos (2015) *Non-parametric representation and prediction of single- and
     multi-shell diffusion-weighted MRI data using Gaussian processes*. NeuroImage **122**:166-176.

src/nifreeze/registration/ants.py

@@ -389,7 +389,7 @@ def generate_command(
             str(p) for p in _massage_mask_path(movingmask_path, nlevels)
         ]
 
-    # Set initalizing affine if provided
+    # Set initializing affine if provided
     if init_affine is not None:
         settings["initial_moving_transform"] = str(init_affine)
 

src/nifreeze/testing/simulations.py

@@ -67,7 +67,7 @@ def add_b0(bvals: np.ndarray, bvecs: np.ndarray) -> tuple[np.ndarray, np.ndarray
 
 def create_single_fiber_evecs(theta: float = 0, phi: float = 0) -> np.ndarray:
     """
-    Create eigenvectors for a simulated fiber given the polar coordinates of its pricipal axis.
+    Create eigenvectors for a simulated fiber given the polar coordinates of its principal axis.
 
     Parameters
     ----------