[api] Correct comment on deprecated v8::String::IsExternal.

Bug: v8:10641
Change-Id: I9248c6877ead34e622b94cd32959b6570f824e15
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390442Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69678}

include/v8.h

@@ -3113,7 +3113,7 @@ class V8_EXPORT String : public Name {
   V8_INLINE static Local<String> Empty(Isolate* isolate);
 
   /**
-   * Returns true if the string is external one-byte.
+   * Returns true if the string is external two-byte.
    *
    */
   V8_DEPRECATE_SOON(