Fix assemble_cvd command

assemble_cvd needs crosvm, but /apex/com.android.virt/bin is not in
$PATH. This adds the directory to $PATH so assemble_cvd can be happy.

Test: manual
Change-Id: Iaf955c67ab92b3bbed04a9a31ece614b0e4a9639
diff --git a/microdroid/README.md b/microdroid/README.md
index e6cbec8..fe0843f 100644
--- a/microdroid/README.md
+++ b/microdroid/README.md
@@ -52,7 +52,7 @@
 future, this shall be done via [`virtmanager`](../virtmanager/).
 
 ```
-$ adb shell 'HOME=/data/local/tmp; /apex/com.android.virt/bin/assemble_cvd < /dev/null'
+$ adb shell 'HOME=/data/local/tmp; PATH=$PATH:/apex/com.android.virt/bin; assemble_cvd < /dev/null'
 $ adb shell 'cd /data/local/tmp; /apex/com.android.virt/bin/crosvm run --cid=5 --disable-sandbox --bios=bootloader --serial=type=stdout --disk=cuttlefish_runtime/composite.img'
 ```