docs/esp32: Fix string quoting consistency in SDCard mount example.

It appears that strings in the documentation are typically single quoted.

docs/esp32/quickref.rst

@@ -589,7 +589,7 @@ See :ref:`machine.SDCard <machine.SDCard>`. ::
 
     # Slot 2 uses pins sck=18, cs=5, miso=19, mosi=23
     sd = machine.SDCard(slot=2)
-    os.mount(sd, "/sd")  # mount
+    os.mount(sd, '/sd')  # mount
 
     os.listdir('/sd')    # list directory contents