doc: Corrected name of argument. (#578)

Author: vtraag

src/_igraph/graphobject.c

@@ -13411,7 +13411,7 @@ struct PyMethodDef igraphmodule_Graph_methods[] = {
  "Generates a tree in which almost all vertices have the same number of children.\n\n"
  "@param n: the number of vertices in the graph\n"
  "@param children: the number of children of a vertex in the graph\n"
- "@param type: determines whether the tree should be directed, and if\n"
+ "@param mode: determines whether the tree should be directed, and if\n"
  " this is the case, also its orientation. Must be one of\n"
  " C{\"in\"}, C{\"out\"} and C{\"undirected\"}.\n"},