fix typo in docstring of variable_upper/lower_bound

Author: yuan-zhou

src/sage/numerical/backends/hybrid_backend.py

@@ -925,7 +925,7 @@ def variable_upper_bound(self, index, value = False):
         - ``index`` (integer) -- the variable's id
 
         - ``value`` -- real value, or ``None`` to mean that the
-          variable has not upper bound. When set to ``None``
+          variable has not upper bound. When set to ``False``
           (default), the method returns the current value.
 
         EXAMPLES::
@@ -962,7 +962,7 @@ def variable_lower_bound(self, index, value = False):
         - ``index`` (integer) -- the variable's id
 
         - ``value`` -- real value, or ``None`` to mean that the
-          variable has no lower bound. When set to ``None``
+          variable has no lower bound. When set to ``False``
           (default), the method returns the current value.
 
         EXAMPLES::