replace U+3BC GREEK SMALL LETTER MU with U+00B5 MICRO SIGN for consistency

I used GREEK SMALL LETTER MU (μ) in the docstring for
_get_design_voltage(), but the rest of the codebase uses MICRO SIGN (µ)
everywhere.

Author: malsyned

src/amd_debug/battery.py

@@ -51,7 +51,7 @@ def _get_design_voltage(self, dev) -> str:
         return ""
 
     def _charge_to_energy(self, dev, charge):
-        """Convert battery charge in μAh to energy in μWh"""
+        """Convert battery charge in µAh to energy in µWh"""
         voltage = self._get_design_voltage(dev)
         if not charge or not voltage:
             return ""