fix: changes positional to named pararameters in Video samples [(#401…

…7)](GoogleCloudPlatform/python-docs-samples#4017)

Changes calls to `VideoClient.annotate_video()` so that GCS URIs are provided as named parameters.

Example:
```
operation = video_client.annotate_video(path, features=features)
```
Becomes:
```
operation = video_client.annotate_video(input_uri=path, features=features)
```

samples/labels/labels.py

@@ -40,7 +40,7 @@ def analyze_labels(path):
     # [START video_label_tutorial_construct_request]
     video_client = videointelligence.VideoIntelligenceServiceClient()
     features = [videointelligence.enums.Feature.LABEL_DETECTION]
-    operation = video_client.annotate_video(path, features=features)
+    operation = video_client.annotate_video(input_uri=path, features=features)
     # [END video_label_tutorial_construct_request]
     print('\nProcessing video for label annotations:')