Backport PR matplotlib#30539: Fix scale_unit/scale_units typo in quiver docs

Author: rcomer

lib/matplotlib/quiver.py

@@ -144,7 +144,7 @@
 
         length in y direction = $\\frac{v}{\\mathrm{scale}} \\mathrm{scale_unit}$
 
-    For example, ``(u, v) = (0.5, 0)`` with ``scale=10, scale_unit="width"`` results
+    For example, ``(u, v) = (0.5, 0)`` with ``scale=10, scale_units="width"`` results
     in a horizontal arrow with a length of *0.5 / 10 * "width"*, i.e. 0.05 times the
     Axes width.