Fix docstring in versions.json

also switched the key to _doc to make it more obvious that it is not a
key to be used.

tools/versions.json

@@ -1,4 +1,4 @@
 {
-    "doc": "This doument is used by the CI to track versions that cannot be tracked by Go.",
+    "_doc": "This document is used by the CI to track versions that cannot be tracked by Go.",
     "flutter-version": "3.13.x"
 }