[FLINK-11173][table] Fix the exception message of proctime attribute validation in TableSourceUtil#validateTableSource

[wenhuitang]

What is the purpose of the change

This pull request makes the exception message of proctime attribute validation in TableSourceUtil#validateTableSource correct.

Brief change log

The exception message of proctime attribute validation in TableSourceUtil#validateTableSource was changed from "Found a RowtimeAttributeDescriptor for " to "Found a processing time attribute: ".

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

[twalthr]

Thank you @wenhuitang. Merging...