doc: fix misleading language in vm docs

The note following the http.Server example in the vm documentation
contains misleading language. This commit removes the incorrect
reference to threads.

Fixes: nodejs#10697
PR-URL: nodejs#10708
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/vm.md

@@ -439,9 +439,9 @@ let code =
  vm.runInThisContext(code)(require);
  ```
 
-*Note*: The `require()` in the above case shares the state with context it is
-passed from. This may introduce risks when untrusted code is executed, e.g.
-altering objects from the calling thread's context in unwanted ways.
+*Note*: The `require()` in the above case shares the state with the context it
+is passed from. This may introduce risks when untrusted code is executed, e.g.
+altering objects in the context in unwanted ways.
 
 ## What does it mean to "contextify" an object?