doc: change Incus_DIR to upper case INCUS_DIR

Signed-off-by: Hiroaki Nakamura <hnakamur@gmail.com>

doc/contributing.md

@@ -37,9 +37,9 @@ git push myfork [name_of_your_new_branch]
 
 ### Important notes for new Incus contributors
 
-- Persistent data is stored in the `Incus_DIR` directory, which is generated by `incus admin init`.
-  The `Incus_DIR` defaults to `/var/lib/incus`.
-- As you develop, you may want to change the `Incus_DIR` for your fork of Incus so as to avoid version conflicts.
+- Persistent data is stored in the `INCUS_DIR` directory, which is generated by `incus admin init`.
+  The `INCUS_DIR` defaults to `/var/lib/incus`.
+- As you develop, you may want to change the `INCUS_DIR` for your fork of Incus so as to avoid version conflicts.
 - Binaries compiled from your source will be generated in the `$(go env GOPATH)/bin` directory by default.
    - You will need to explicitly invoke these binaries (not the global `incusd` you may have installed) when testing your changes.
    - You may choose to create an alias in your `~/.bashrc` to call these binaries with the appropriate flags more conveniently.