Fix typos in docs

NeST documentation is maintained at https://nest.nitk.ac.in/docs/master/index.html. These docs help new NeST users to quickly get up to speed with using NeST. Inadvertently, some typos have slipped into the documentation. Below are some typos in Architecture section of the docs (marked in red):

The source code for this section is here: https://gitlab.com/nitk-nest/nest/-/blob/master/docs/source/developer/architecture.rst.

Fix the typos in this file (and also in other documentation files, if there are any!)