Wrong docs of behavior test for setting up .env

[NKID00]

Docs of behavior test (and also docs of fuzzer) gives an example to set up .env with OPENDAL_FS_ROOT=/tmp, but tests would fail if .env was set up according to it with error as follows:

thread 'main' panicked at src/raw/tests/utils.rs:70:45:
must succeed: Unexpected (permanent) at Builder::build => create root dir failed

Context:
   root: /tmp162b96c0-ac99-44ac-89cf-839db11dce66/

Source:
   Permission denied (os error 13)

This is due to that the given root and random uuid were glued together without a slash.

Maybe we should change the docs to /tmp/ or to utilize Path::join to generate random roots.

[Xuanwo]

Maybe we should change the docs to /tmp/

Let's fix the docs. Thanks for the catch!