chore: fix incorrect role_bindings var description

[bhegazy]

Description

Fix incorrect description for role_bindings variable in variables.tf.

I started to use the module today and found out this copy/paste error

Changes

• Updated the description of the role_bindings variable to accurately reflect its purpose
• The previous description was incorrectly copied from max_request_duration_hours variable

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[imrannayer]

/gcbrun

[imrannayer]

@bhegazy thanks for the PR. Can you plz submit CLA. Was sent to your bil******zy​@gmail.com address

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