[typo] fix language_data.js_t (#12228)

Co-authored-by: Bénédikt Tran <[email protected]>

sphinx/themes/basic/static/language_data.js_t

@@ -13,7 +13,7 @@
 var stopwords = {{ search_language_stop_words }};
 
 {% if search_language_stemming_code %}
-/* Non-minified version is copied as a separate JS file, is available */
+/* Non-minified version is copied as a separate JS file, if available */
 {{ search_language_stemming_code|safe }}
 {% endif -%}