Rename sd->sigmadelta in HAL

Change is to not be mistaken with SD Card

Author: me-no-dev

README.md

@@ -16,7 +16,7 @@
 ## Development Status
 Most of the framework is implemented. Most noticable is the missing analogWrite. While analogWrite is on it's way, there are a few other options that you can use:
 - 16 channels [LEDC](https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-ledc.h) which is PWM
-- 8 channels [SigmaDelta](https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-sd.h) which uses SigmaDelta modulation
+- 8 channels [SigmaDelta](https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-sigmadelta.h) which uses SigmaDelta modulation
 - 2 channels [DAC](https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-dac.h) which gives real analog output
 
 ## Installation Instructions