vocab : fix string init

Co-authored-by: Diego Devesa <[email protected]>

Author: ggerganov

src/llama-vocab.cpp

@@ -2456,7 +2456,7 @@ std::vector<llama_token> llama_vocab::tokenize(
 
                         // prefix with space if previous is special
                         if (tokenizer_add_space_prefix && is_prev_special) {
-                            text += ' ';
+                            text = ' ';
                         }
 
                         text += fragment.raw_text.substr(fragment.offset, fragment.length);