LibWeb: Fix highlighting HTML comments

Commit b193351 caused the HTML comments to flash when changing
the text cursor. Also, when double-clicking on a comment, the selection
started from the beginning of the file instead.

The following message was displaying when `TOKENIZER_TRACE_DEBUG`
was enabled:

    (Tokenizer::nth_last_position) Invalid position requested: 4th-last
    of 4. Returning (0-0).

Changing the `nth_last_position` to 3 fixes this. I'm guessing that's
because the parser is at that moment on the second hyphen of the `<!--`
string, so it has to go back only by three characters.

Tests/LibWeb/TestHTMLTokenizer.cpp

@@ -206,5 +206,5 @@ TEST_CASE(regression)
  auto file_contents = file.value()->read_all();
  auto tokens = run_tokenizer(file_contents);
  u32 hash = hash_tokens(tokens);
- EXPECT_EQ(hash, 1280197787u);
+ EXPECT_EQ(hash, 3215459107u);
 }

Userland/Libraries/LibWeb/HTML/Parser/HTMLTokenizer.cpp

@@ -382,7 +382,7 @@ Optional<HTMLToken> HTMLTokenizer::next_token()
  DONT_CONSUME_NEXT_INPUT_CHARACTER;
  if (consume_next_if_match("--")) {
  create_new_token(HTMLToken::Type::Comment);
- m_current_token.set_start_position({}, nth_last_position(4));
+ m_current_token.set_start_position({}, nth_last_position(3));
  SWITCH_TO(CommentStart);
  }
  if (consume_next_if_match("DOCTYPE", CaseSensitivity::CaseInsensitive)) {