Use correct color in comment

Author: yvonnefroehlich

book/tut4_geophysics.ipynb

@@ -1101,7 +1101,7 @@
     "fig.plot(\n",
     "    x=track_df[\"p\"],\n",
     "    y=track_df[\"elevation\"],\n",
-    "    fill=\"lightgray\",  # Fill the polygon in \"gray\"\n",
+    "    fill=\"lightgray\",  # Fill the polygon in \"lightgray\"\n",
     "    # Draw a 1-point thick, black, solid outline\n",
     "    pen=\"1p,black,solid\",\n",
     "    close=\"+y-7500\",  # Force closed polygon\n",