esp32/machine_sdcard: Fix bug in SPI slot number selection.

And fix minor typo in docs when referring to SDCard class.

docs/library/machine.SDCard.rst

@@ -30,7 +30,7 @@ vary from platform to platform.
     The class implements the block protocol defined by :class:`uos.AbstractBlockDev`.
     This allows the mounting of an SD card to be as simple as::
 
-      uos.mount(storage.SDCard(), "/sd")
+      uos.mount(machine.SDCard(), "/sd")
 
     The constrcutor takes the following paramters:
 

ports/esp32/machine_sdcard.c

@@ -203,7 +203,6 @@ STATIC mp_obj_t machine_sdcard_make_new(const mp_obj_type_t *type, size_t n_args
 
     if (is_spi) {
         self->host.slot = slot_num ? HSPI_HOST : VSPI_HOST;
-        slot_num -= 2;
     }
 
     DEBUG_printf("  Calling host.init()");