Update instructions for `jiri import` to include --name flag

Without this flag, `jiri update` will complain about being unable to
resolve remote import: project "=https://foo/mojo-manifest"

Change-Id: Ib3c00a5e73d87df257252cfecf547cdb51249da3

README.md

diff --git a/README.md b/README.md
index 159b2fb..a735a83 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,7 @@
 curl -s https://raw.githubusercontent.com/vanadium/go.jiri/master/scripts/bootstrap_jiri | bash -s mojo
 export PATH=`pwd`/mojo/.jiri_root/scripts:`pwd`/mojo/depot_tools:$PATH
 cd mojo
-jiri import mojo https://mojo.googlesource.com/mojo-manifest
+jiri import --name manifest mojo https://mojo.googlesource.com/mojo-manifest
 jiri update
 ```