Fix typo

Author: Amar3tto

sdks/python/apache_beam/examples/ml_transform/mltransform_generate_vocab_requirements.txt

@@ -18,7 +18,7 @@
 # MLTransform TFT operations need a consistent TensorFlow Transform stack;
 # otherwise workers can crash-loop with pandas/numpy ABI mismatches.
 google-cloud-monitoring>=2.27.0
-tensorflow_transform=1.21.0
+tensorflow_transform==1.21.0
 tensorflow-metadata==1.21.0
 tfx-bsl==1.21.0
 dill