doc: installing-usb make macOS note a note.

While it seemingly brings more attention to the macOS notes with the
default docbook template, it better represents which parts of the
section are about macOS, and which parts are simply in the flow of the
text; otherwise the last paragraph may be lost into the details for
macOS.

(cherry picked from commit 8192fcd)

nixos/doc/manual/installation/installing-usb.xml

@@ -9,13 +9,12 @@
   For systems without CD drive, the NixOS live CD can be booted from a USB
   stick. You can use the <command>dd</command> utility to write the image:
   <command>dd if=<replaceable>path-to-image</replaceable>
-  of=<replaceable>/dev/sdb</replaceable></command>. Be careful about specifying
+  of=<replaceable>/dev/sdX</replaceable></command>. Be careful about specifying
   the correct drive; you can use the <command>lsblk</command> command to get a
   list of block devices.
- </para>
-
- <para>
-  On macOS:
+  <note>
+   <title>On macOS</title>
+   <para>
 <programlisting>
 $ diskutil list
 [..]
@@ -26,10 +25,12 @@ $ diskutil unmountDisk diskN
 Unmount of all volumes on diskN was successful
 $ sudo dd bs=1m if=nix.iso of=/dev/rdiskN
 </programlisting>
-  Using the 'raw' <command>rdiskN</command> device instead of
-  <command>diskN</command> completes in minutes instead of hours. After
-  <command>dd</command> completes, a GUI dialog "The disk you inserted was not
-  readable by this computer" will pop up, which can be ignored.
+    Using the 'raw' <command>rdiskN</command> device instead of
+    <command>diskN</command> completes in minutes instead of hours. After
+    <command>dd</command> completes, a GUI dialog "The disk you inserted was
+    not readable by this computer" will pop up, which can be ignored.
+   </para>
+  </note>
  </para>
 
  <para>