Small error in documentation: "[...] where x' has the same size as x [...]"

Change-Id: I09d6c78743229a4047dbc8d21f148f0e36fefdc0

docs/source/modeling.rst

diff --git a/docs/source/modeling.rst b/docs/source/modeling.rst
index 1083851..a34e696 100644
--- a/docs/source/modeling.rst
+++ b/docs/source/modeling.rst
@@ -818,7 +818,7 @@
 
    .. math:: x' = \boxplus(x, \Delta x),
 
-   where :math:`x` has the same size as :math:`x`, and :math:`\Delta
+   where :math:`x'` has the same size as :math:`x`, and :math:`\Delta
    x` is of size less than or equal to :math:`x`. The function
    :math:`\boxplus`, generalizes the definition of vector
    addition. Thus it satisfies the identity