docs: Use json to fix syntax highlighting (#17884)

This follows up the [issue with mdbook notpeter
mentioned](https://github.com/zed-industries/zed/pull/17864#issuecomment-2353089065)
by replacing `jsonc` where used in the docs with `json`.

Additionally, one missing `json` - highlight was added for the
search-section.

Release Notes:

- N/A

docs/src/languages/csharp.md

@@ -11,15 +11,15 @@ C# support is available through the [C# extension](https://github.com/zed-indust
 
 The `OmniSharp` binary can be configured in a Zed settings file with:
 
-```jsonc
+```json
 {
   "lsp": {
     "omnisharp": {
       "binary": {
         "path": "/path/to/OmniSharp",
-        "args": ["optional", "additional", "args", "-lsp"],
-      },
-    },
-  },
+        "args": ["optional", "additional", "args", "-lsp"]
+      }
+    }
+  }
 }
 ```