DOC: Add negative to equation in the advection docs

This helps make clear that we're returning -u dot del and matches the
implementation.

src/metpy/calc/kinematics.py

@@ -434,8 +434,8 @@ def advection(
     -----
     This implements the advection of a scalar quantity by wind:
 
-    .. math:: \mathbf{u} \cdot \nabla = u \frac{\partial}{\partial x}
-              + v \frac{\partial}{\partial y} + w \frac{\partial}{\partial z}
+    .. math:: -\mathbf{u} \cdot \nabla = -(u \frac{\partial}{\partial x}
+              + v \frac{\partial}{\partial y} + w \frac{\partial}{\partial z})
 
     .. versionchanged:: 1.0
        Changed signature from ``(scalar, wind, deltas)``