DOC: Correct docstring for gradient_richardson_number

Based on some reading, it doesn't seem like our implementation is
strictly equivalent to flux Richardson number, which seems to rely on
prototypical boundarly layer perturbations rather than gradients. To be
safe, let's just leave flux out of the docstring.

src/metpy/calc/thermo.py

@@ -3323,7 +3323,7 @@ def k_index(pressure, temperature, dewpoint):
 )
 @check_units('[length]', '[temperature]', '[speed]', '[speed]')
 def gradient_richardson_number(height, potential_temperature, u, v, vertical_dim=0):
-    r"""Calculate the gradient (or flux) Richardson number.
+    r"""Calculate the gradient Richardson number.
 
     .. math:: Ri = \frac{g}{\theta} \frac{\left(\partial \theta/\partial z\right)}
              {\left(\partial u / \partial z\right)^2 + \left(\partial v / \partial z\right)^2}