Fix missing assignment (#3151)

Co-authored-by: Quentin Pradet <[email protected]>

Author: celeste0-0sec

elasticsearch/esql/esql.py

@@ -140,7 +140,7 @@ def _format_id(id: FieldType, allow_patterns: bool = False) -> str:
         if re.fullmatch(r"[a-zA-Z_@][a-zA-Z0-9_\.]*", s):
             return s
         # this identifier needs to be escaped
-        s.replace("`", "``")
+        s = s.replace("`", "``")
         return f"`{s}`"
 
     @staticmethod