Fix installation instructions for installing profile packages.

The step-by-step install instructions tell the user to run `jiri profile
os-packages v23:base`, and then run the output as sudo.  This works for
the step-by-step instructions, but not for the main installation script,
which is concatenation of the step-by-steps.

This CL adds an explicit line "sudo $(jiri profile os-packages
v23:base)" to the step-by-step instructions that will install the
package prereqs, fixing the main installation script.

Fixes https://github.com/vanadium/issues/issues/1346

Change-Id: I32188e92a6e15bf8b28f38a8f0062609a896488c
diff --git a/content/installation/step-by-step.md b/content/installation/step-by-step.md
index 577641d..c0ea862 100644
--- a/content/installation/step-by-step.md
+++ b/content/installation/step-by-step.md
@@ -123,7 +123,11 @@
 
 <!-- @packagesBaseProfile @test -->
 ```
+# Print the package installation command.
 jiri profile os-packages v23:base
+
+# Run the package installation command as root.
+sudo $(jiri profile os-packages v23:base)
 ```
 
 Then install the prerequisites themselves.