chore(group_theory/nielsen_schreier): typos in module doc-string (#7500)

This fixes a discrepancy between the doc-string and the rest of the file.

src/group_theory/nielsen_schreier.lean

@@ -23,9 +23,9 @@ but we work directly with groupoids instead of topological spaces. Under this an
 - `is_free_groupoid G` corresponds to saying that a space is a graph.
 - `End_mul_equiv_subgroup H` plays the role of replacing 'subgroup of fundamental group' with
   'fundamental group of covering space'.
-- `action_category_is_free G A` corresponds to the fact that a covering of a (single-vertex)
+- `action_groupoid_is_free G A` corresponds to the fact that a covering of a (single-vertex)
   graph is a graph.
-- `End_is_free_group_of_arborescence` corresponds to the fact that, given a spanning tree of a
+- `End_is_free T` corresponds to the fact that, given a spanning tree `T` of a
   graph, its fundamental group is free (generated by loops from the complement of the tree).
 
 ## Implementation notes