Manual: the “expression” option of “var_expectation_model” does not take a string

[skip ci]

(cherry picked from commit 659474bd)

doc/manual/source/the-model-file.rst

@@ -13082,9 +13082,9 @@ More generally, it is possible to consider finite discounted sums.
     Name of the associated auxiliary model, defined with ``var_model`` or ``trend_component_model``.
-    .. option:: expression = STRING | QUOTED_STRING
-    Name of the variable or expression (linear combination of variables) to be expected. The quotes are mandatory if an expression is expected.
+    .. option:: expression = VARIABLE_NAME | EXPRESSION
+    Name of the variable or expression (linear combination of variables) to be expected.
     .. option:: discount = PARAMETER_NAME | DOUBLE