Updated Sphinx configuration instructions

Hi,

While starting contributing, I ran into an error in Sphinx configuration.
The syntax of the configuration file should be slightly modified, here I propose to add a "tip", for users who may encounter the same issue.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | kinda: added a tip
| Applies to    | N/A
| Fixed tickets | none