[docs] Fix build break due to missing comma (#4561)

Author: kiennq

docs/lsp-clients.json

@@ -338,7 +338,7 @@
     "name": "futhark",
     "full-name": "Futhark",
     "server-name": "futhark-lsp",
-    "server-url": "https://github.com/diku-dk/futhark"
+    "server-url": "https://github.com/diku-dk/futhark",
     "installation": "A part of the compiler since 0.21.9",
     "debugger": "Not available"
   },