Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>

Author: williamhobbs

pvlib/pvsystem.py

@@ -2893,7 +2893,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     This function accepts an optional irradiance adjustment factor, `k`, based
     on [2]_. This applies a piece-wise adjustment to power based on irradiance,
-    where `k` is the reduction in actual power at 200 W/m^2 relative to power
+    where `k` is the reduction in actual power at 200 Wm⁻² relative to power
     calculated at 200 W/m^2 as 0.2*`pdc0`. For example, a 500 W module that
     produces 95 W at 200 W/m^2 (a 5% relative reduction in efficiency) would
     have a value of `k` = 0.01.