Fix NU_LIB_PATH -> NU_LIB_DIRS

`$env.NU_LIB_PATH` does not exist

In the block that follows the correct name is being used.

Author: Kissaki

book/modules/using_modules.md

@@ -69,7 +69,7 @@ The path to the module can be:
   Note that the module name (its directory) can end in a `/` (or `\` on Windows), but as with most commands that take a paths (e.g., `cd`), this is completely optional.
   :::
 
-  ::: important Important! Importing modules from `$env.NU_LIB_PATH`
+  ::: important Important! Importing modules from `$env.NU_LIB_DIRS`
   When importing a module via a relative path, Nushell first searches from the current directory. If a matching module is not found at that location, Nushell then searches each directory in the `$env.NU_LIB_DIRS` list.
 
   This allows you to install modules to a location that is easily accessible via a relative path regardless of the current directory.