Documentation references old branches

[queeniema]

Repositories in the IBMStreams organization have been updated to remove/rename the master branch. References to this old branch need to be updated:

Example:

https://github.com/IBMStreams/streamsx.messaging/tree/master/samples/KafkaSample
should be:
https://github.com/IBMStreams/streamsx.messaging/tree/develop/samples/KafkaSample

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Bootstrap has also made similar changes and main is now the default branch.

Example:

https://github.com/twbs/bootstrap/blob/master/LICENSE
should be:
https://github.com/twbs/bootstrap/blob/main/LICENSE