Add new topic item 'subplot' to recipe filter keyword list

Author: sadielbartholomew

docs/source/recipes/recipe_list.txt

@@ -13,7 +13,7 @@ plot_06_recipe.html#sphx-glr-recipes-plot-06-recipe-py
 plot_07_recipe.html#sphx-glr-recipes-plot-07-recipe-py
 <div class="sphx-glr-thumbcontainer aggregate lineplot subspace" tooltip="Aggregate, Lineplot, Subspace">
 plot_08_recipe.html#sphx-glr-recipes-plot-08-recipe-py
-<div class="sphx-glr-thumbcontainer collapse contourmap" tooltip="Collapse, Contourmap">
+<div class="sphx-glr-thumbcontainer collapse contourmap" tooltip="Collapse, Contourmap, Subplot">
 plot_09_recipe.html#sphx-glr-recipes-plot-09-recipe-py
 <div class="sphx-glr-thumbcontainer histogram" tooltip="Histogram">
 plot_10_recipe.html#sphx-glr-recipes-plot-10-recipe-py
@@ -27,6 +27,6 @@ plot_13_recipe.html#sphx-glr-recipes-plot-13-recipe-py
 plot_14_recipe.html#sphx-glr-recipes-plot-14-recipe-py
 <div class="sphx-glr-thumbcontainer subspace collapse contourmap" tooltip="Subspace, Collapse, Contourmap">
 plot_15_recipe.html#sphx-glr-recipes-plot-15-recipe-py
-<div class="sphx-glr-thumbcontainer histogram subspace" tooltip="Histogram, Subspace">
+<div class="sphx-glr-thumbcontainer histogram subspace" tooltip="Histogram, Subspace, Subplot">
 plot_16_recipe.html#sphx-glr-recipes-plot-16-recipe-py
-<div class="sphx-glr-thumbcontainer histogram subspace" tooltip="Contourmap, Subspace">
+<div class="sphx-glr-thumbcontainer histogram subspace" tooltip="Contourmap, Subspace, Subplot">