fix typo in docstring

Author: yyyyx4

src/sage/schemes/elliptic_curves/hom_composite.py

@@ -773,7 +773,7 @@ def is_injective(self):
         In other words, return ``True`` if and only if ``self`` is a
         purely inseparable isogeny.
 
-        EXAMPLE::
+        EXAMPLES::
 
             sage: from sage.schemes.elliptic_curves.hom_composite import EllipticCurveHom_composite
             sage: E = EllipticCurve([1,0])