Update gruvbox.md

Author: tedbyron

docs/color_theming/gruvbox.md

@@ -8,7 +8,7 @@ white='#EBDBB2'
 gray='#32302F'
 dark_gray='#282828'
 light_purple='#B16286'
-dark_purple='#3C3836'
+dark_purple='#504945'
 cyan='#689D6A'
 green='#98971A'
 orange='#D65D0E'