fix user_guide sample command

[e-koma]

The command in the sample Cloud Shell is incorrect and I fixed it.

This Cloud Shell's current path is already in the policy-library repository.

$ pwd
/home/yuto_komai/cloudshell_open/policy-library
$ 
$ cp policy-library/samples/iam_service_accounts_only.yaml policy-library/policies/constraints
cp: cannot stat 'policy-library/samples/iam_service_accounts_only.yaml': No such file or directory

if fixed it

$ cp samples/iam_service_accounts_only.yaml policies/constraints 
$ 
$ ls -la policies/constraints
total 20
drwxr-xr-x 2 yuto_komai yuto_komai 4096 Feb 10 06:59 .
drwxr-xr-x 4 yuto_komai yuto_komai 4096 Feb 10 06:57 ..
-rw-r--r-- 1 yuto_komai yuto_komai  531 Feb 10 06:59 iam_service_accounts_only.yaml
-rw-r--r-- 1 yuto_komai yuto_komai  473 Feb 10 06:57 Kptfile
-rw-r--r-- 1 yuto_komai yuto_komai  193 Feb 10 06:57 README.md