fix bs4 warning on text kwarg - switch to string

Fix warning as proposed by bs4:
```
  .../bs4/element.py:2253: DeprecationWarning: The 'text' argument to find()-type methods is deprecated. Use 'string' instead.
    return self.find_all(
```

crawl4ai/content_filter_strategy.py

@@ -656,7 +656,7 @@ class PruningContentFilter(RelevantContentFilter):
 
     def _remove_comments(self, soup):
         """Removes HTML comments"""
-        for element in soup(text=lambda text: isinstance(text, Comment)):
+        for element in soup(string=lambda string: isinstance(string, Comment)):
             element.extract()
 
     def _remove_unwanted_tags(self, soup):