docs(config): fix temp_dir default value

Ref: #2930

Author: César Román

docs/config.rst

@@ -174,7 +174,7 @@ Core
 
 .. conf::
    :keys: temp_dir
-   :default: {tox_root}/.tmp
+   :default: {work_dir}/.tmp
 
    Directory where to put tox temporary files. For example: we create a hard link (if possible, otherwise new copy) in
    this directory for the project package. This ensures tox works correctly when having parallel runs (as each session