Pluralize "syntax diagrams"

These buttons show/hide multiple diagrams (all diagrams in the book,
really), so it makes sense it probably should be plural.

Closes https://github.com/rust-lang/reference/issues/1842
diff --git a/theme/reference.js b/theme/reference.js
index 599b662..5fdf6f1 100644
--- a/theme/reference.js
+++ b/theme/reference.js
@@ -66,9 +66,9 @@
     const buttons = document.querySelectorAll('.grammar-toggle-railroad');
     buttons.forEach(button => {
         if (grammarRailroad) {
-            button.innerText = "Hide syntax diagram";
+            button.innerText = "Hide syntax diagrams";
         } else {
-            button.innerText = "Show syntax diagram";
+            button.innerText = "Show syntax diagrams";
         }
     });
 }