Use maxdepth=3 in the API index (#87)

This makes the API index page list every function in the spec.

Author: asmeurer

spec/API_specification/index.rst

@@ -5,7 +5,7 @@ API specification
 
 .. toctree::
    :caption: API specification
-   :maxdepth: 1
+   :maxdepth: 3
 
    function_and_method_signatures
    array_object
@@ -22,4 +22,4 @@ API specification
    sorting_functions
    set_functions
    utility_functions
-   constants
+   constants