community: fix `DirectoryLoader` progress bar (#19821)

**Description:** currently, the `DirectoryLoader` progress-bar maximum value is based on an incorrect number of files to process

In langchain_community/document_loaders/directory.py:127:

```python
        paths = p.rglob(self.glob) if self.recursive else p.glob(self.glob)
        items = [
            path
            for path in paths
            if not (self.exclude and any(path.match(glob) for glob in self.exclude))
        ]
```

`paths` returns both files and directories. `items` is later used to determine the maximum value of the progress-bar which gives an incorrect progress indication.

libs/community/langchain_community/document_loaders/directory.py

@@ -129,6 +129,7 @@ class DirectoryLoader(BaseLoader):
             path
             for path in paths
             if not (self.exclude and any(path.match(glob) for glob in self.exclude))
+            and path.is_file()
         ]
 
         if self.sample_size > 0: