Fix iam.martin_ruiz equation in docs #1561 (#1599)

* Fix martin_ruiz eq in docs #1561

* Add whatsnew entry

Author: echedey-ls

docs/sphinx/source/whatsnew/v0.9.4.rst

@@ -27,9 +27,10 @@ Testing
 ~~~~~~~
 * Corrected a flawed test for :py:func:`~pvlib.irradiance.get_ground_diffuse` (:issue:`1569`, :pull:`1575`)
 
+
 Documentation
 ~~~~~~~~~~~~~
-
+* Fixed equation in :py:func:`pvlib.iam.martin_ruiz` docstring (:issue:`1561`, :pull:`1599`)
 
 Benchmarking
 ~~~~~~~~~~~~~
@@ -48,3 +49,4 @@ Contributors
 * Marcus Boumans (:ghuser:`bowie2211`)
 * Karel De Brabandere (:ghuser:`kdebrab`)
 * Naman Priyadarshi (:ghuser:`Naman-Priyadarshi`)
+* Echedey Luis (:ghuser:`echedey-ls`)

pvlib/iam.py

@@ -227,7 +227,7 @@ def martin_ruiz(aoi, a_r=0.16):
 
     .. math::
 
-       IAM = \frac{1 - \exp(-\cos(\frac{aoi}{a_r}))}
+       IAM = \frac{1 - \exp(-\frac{\cos(aoi)}{a_r})}
        {1 - \exp(\frac{-1}{a_r})}
 
     which is presented as :math:`AL(\alpha) = 1 - IAM` in equation 4 of [1]_,
@@ -353,7 +353,6 @@ def martin_ruiz_diffuse(surface_tilt, a_r=0.16, c1=0.4244, c2=None):
     # avoid undefined results for horizontal or upside-down surfaces
     zeroang = 1e-06
 
-
     surface_tilt = np.where(surface_tilt == 0, zeroang, surface_tilt)
     surface_tilt = np.where(surface_tilt == 180, 180 - zeroang, surface_tilt)