Manual:$wgNamespaceAliases

Details
These are alternate names for the primary localised namespace names, which are defined by $wgExtraNamespaces and the language file. If a page is requested with such a prefix, the request will be redirected to the primary name.

Set this to a map from namespace names to IDs. Example:

To add a single alias: If you need an alias for the project you probably need a similar alias for its talk page and you should add an array as above or two lines:

Do not use spaces
Use underscores instead of spaces in namespace names. 'My Namespace' is an invalid name; use 'My_Namespace' instead.