[Content Improvement] - update deprecated functions

**Link to the notebook**
[blazingtext_text_classification_dbpedia.ipynb
](https://github.com/aws/amazon-sagemaker-examples/blob/e6b96e3a7875cf4ea5c830608e27891c159791d2/introduction_to_amazon_algorithms/blazingtext_text_classification_dbpedia/blazingtext_text_classification_dbpedia.ipynb), presumably others with references to deprecated functions

**What aspects of the notebook can be improved?**
Update deprecated function calls to use their replacements:

**What are your suggestions?**

- replace `sagemaker.amazon_estimator.get_image_uri()` with `sagemaker.image_uris.retrieve()`

Open introduction_to_amazon_algorithms/blazingtext_text_classification_dbpedia/blazingtext_text_classification_dbpedia.ipynb and locate the deprecated sagemaker.amazon_estimator.get_image_uri() calls shown in the issue. Review nearby SageMaker setup before replacing them with sagemaker.image_uris.retrieve(), then verify that the notebook no longer contains the deprecated calls; check whether other referenced notebooks require the same update.

Written by the indexing model from the issue text.