Fix raster-functions.rst doc

[RickLeite]

just some small fixes.

Changes Made:

• Modifications for inconsistency in rst_isempty, rst_memsize, and rst_numbands functions.
• Added some missing parentheses.

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}