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

davepeake · dpgeorge · commit 454d96978156 · 2022-08-17T12:00:35.000+10:00
It appears that strings in the documentation are typically single quoted.
diff --git a/docs/esp32/quickref.rst b/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
 
