[fix] categories: rename stock_images category to stock images (#6760)

Category ``stock_images`` is referenced to as ``stock images`` everywhere else,
so SXNG didn't find any translation for ``stock_images``.

searx/settings.yml

@@ -280,7 +280,7 @@ plugins:
 categories_as_tabs:
   general:
   images:
-  # stock_images:
+  # stock images:
   videos:
   news:
   map:
@@ -1932,7 +1932,7 @@ engines:
   - name: pixabay images
     engine: pixabay
     pixabay_type: images
-    categories: stock_images
+    categories: stock images
     shortcut: pixi
     disabled: true