fix docstring so that case reflects string returned in ecg_quality

Author: danibene

neurokit2/ecg/ecg_quality.py

@@ -57,7 +57,7 @@ def ecg_quality(
     -------
     array or str
         Vector containing the quality index ranging from 0 to 1 for ``"averageQRS"`` method,
-        returns string classification (``Unacceptable``, ``Barely Acceptable`` or ``Excellent``)
+        returns string classification (``Unacceptable``, ``Barely acceptable`` or ``Excellent``)
         of the signal for ``"zhao2018"`` method.
 
     See Also