Properly quote <username>

That way, it appears in the right font, *and* it's not
interpreted as markup.

Change-Id: I080ce06fb1c2e5c017d625510cf20fb1f30471ad
diff --git a/src/source/initializing.md b/src/source/initializing.md
index 6a29c3c..7f07728 100644
--- a/src/source/initializing.md
+++ b/src/source/initializing.md
@@ -121,7 +121,7 @@
 
 The recommended approach is to create a file
 `/etc/udev/rules.d/51-android.rules` (as the root user) and to copy
-the following lines in it. <username> must be replaced by the
+the following lines in it. `<username>` must be replaced by the
 actual username of the user who is authorized to access the phones
 over USB.