doc/icE1usb: fix warning about two list items '1'

asciidoc: WARNING: firmware.adoc: line 25: list item index: expected 2 got 1
Change-Id: I7312538c6aa723e25176a6a5288ccab6c03e5f08
diff --git a/doc/manuals/chapters/firmware.adoc b/doc/manuals/chapters/firmware.adoc
index 5e4a11c..069a2ba 100644
--- a/doc/manuals/chapters/firmware.adoc
+++ b/doc/manuals/chapters/firmware.adoc
@@ -22,7 +22,7 @@
    (obviously that requires a [still] working firmware present on the
    device).  To do so, please use `dfu-util -e`
 
-1. by pushing the push-button (see <<hw-pushbutton>>) during power-up.
+2. by pushing the push-button (see <<hw-pushbutton>>) during power-up.
    Simply disconnect the USB cable, then push that button and keep it
    pushed while re-attaching the USB cable.