sagemathgh-40551: add missing double colon
    
Sphinx needs a double colon befor an indented block (of example code).
    
URL: sagemath#40551
Reported by: Martin Rubey
Reviewer(s): David Coudert

Author: Release Manager

src/sage/combinat/free_module.py

@@ -672,7 +672,7 @@ def _element_constructor_(self, x):
             sage: F = CombinatorialFreeModule(QQ,[0,1])
 
         Is ``0`` the zero of the base ring, or the index of a basis
-        element?  I.e. should the result be ``0`` or ``B[0]``?
+        element?  I.e. should the result be ``0`` or ``B[0]``?::
 
             sage: F = CombinatorialFreeModule(QQ,[0,1])
             sage: F(0) # this feature may eventually disappear