Fix getting max magnitude in AudioEffectSpectrumAnalyzer

(cherry picked from commit 47f628a013e501b9eefb7ed8d5b23862bc5083ea)

servers/audio/effects/audio_effect_spectrum_analyzer.cpp

@@ -170,7 +170,7 @@ Vector2 AudioEffectSpectrumAnalyzerInstance::get_magnitude_for_frequency_range(f
 
 		for (int i = begin_pos; i <= end_pos; i++) {
 			max.x = MAX(max.x, r[i].l);
-			max.y = MAX(max.x, r[i].r);
+			max.y = MAX(max.y, r[i].r);
 		}
 
 		return max;