fix doctest with igraph

[dcoudert]

Fix the doctest error reported in #27480 comment:78 with a tolerance gap

**********************************************************************
File "src/sage/graphs/generic_graph.py", line 9663, in sage.graphs.generic_graph.GenericGraph.?
Failed example:
    G.pagerank(alpha=0.50, algorithm="igraph")  # optional - python_igraph
Expected:
    {0: 0.25, 1: 0.25, 2: 0.24999999999999997, 3: 0.24999999999999997}
Got:
    {0: 0.25, 1: 0.25, 2: 0.25, 3: 0.25}
**********************************************************************
1 item had failures:
   1 of 860 in sage.graphs.generic_graph.GenericGraph.?
    [3497 tests, 1 failure, 50.96 s]
----------------------------------------------------------------------
sage -t --long src/sage/graphs/generic_graph.py  # 1 doctest failed

CC: @fchapoton

Component: graph theory

Keywords: igraph

Author: David Coudert

Branch/Commit: 2422888

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/27811

[dcoudert]

Commit: 2422888

[dcoudert]

New commits:

2422888

trac #27811: fix doctest with igraph

[dcoudert]

Branch: public/graphs/27811_igraph

[fchapoton]

comment:2

ok, thx

[fchapoton]

Reviewer: Frédéric Chapoton

[vbraun]

Changed branch from public/graphs/27811_igraph to 2422888