Fix typo in show_whitespaces docs in default setting (#13735)

Release Notes:

- N/A

assets/settings/default.json

@@ -166,7 +166,7 @@
   // 3. Draw all invisible symbols:
   //    "all"
   // 4. Draw whitespaces at boundaries only:
-  //   "boundaries"
+  //    "boundary"
   // For a whitespace to be on a boundary, any of the following conditions need to be met:
   // - It is a tab
   // - It is adjacent to an edge (start or end)

docs/src/configuring-zed.md

@@ -1123,7 +1123,7 @@ These values take in the same options as the root-level settings with the same n
 1. `all`
 2. `selection`
 3. `none`
-4. `boundaries`
+4. `boundary`
 
 ## Soft Wrap