🎨 style(data_processing.py): fix typo in variable names

The variable names σ_〈R〉 and σ_〈R〉^tot were corrected to σ_<R> and σ_<R>^tot respectively to improve consistency with the naming conventions used in the rest of the code.

utils/data_processing/data_processing.py

@@ -123,8 +123,8 @@ def get_advanced_data(discrete_data:dict, V_scan = 0, update_progress_bar= progr
         - Volume en mm3
         - Surface en mm2
         - Moyenne des rayons moyens 〈R〉
-        - Ecart-type des rayons moyens σ_〈R〉
-        - σ_〈R〉^tot
+        - Ecart-type des rayons moyens σ_<R>
+        - σ_<R>^tot
         - H
         - L
         - l