Fix typo in scheme.py

Author: r-mb

src/sage/schemes/generic/scheme.py

@@ -120,7 +120,7 @@ def __init__(self, X=None, category=None):
             # X is a morphism of Rings
             self._base_ring = X.codomain()
         else:
-            raise ValueError('The base must be define by a scheme, '
+            raise ValueError('The base must be defined by a scheme, '
                              'scheme morphism, or commutative ring.')
 
         from sage.categories.schemes import Schemes