sage.graphs.graph: fix typo "Peterson"

Author: orlitzky

src/sage/graphs/graph.py

@@ -9567,7 +9567,7 @@ def is_projective_planar(self, return_map=False):
 
         EXAMPLES:
 
-        The Peterson graph is a known projective planar graph::
+        The Petersen graph is a known projective planar graph::
 
             sage: P = graphs.PetersenGraph()
             sage: P.is_projective_planar()