Fix typos and improve clarity in README.md and CONTRIBUTING.md

[A-RAKSHITHA]

1. Changes in README.md:
   Typo in "Contributions" Section:

Old: "We are managing contributions through pull requents here on GitHub."

New: "We are managing contributions through pull requests here on GitHub."

Change: Corrected the typo "requents" to "requests."

Clarity Improvements in the "Contributions" Section:

Old: "please make master the target branch."

New: "please set the target branch to master when creating your pull request."

Change: Made the instruction clearer.

Old: "New features should be created in feature/branches and be based on master."

New: "New features should be developed in feature/ branches, based on the master branch."

Change: Improved the wording for clarity.

2. Changes in CONTRIBUTING.md:
   Incomplete Sentence Fix:

Old (incomplete): "SAMRAI's tests are all in the /source/test and are spilt up by"

New (completed): "SAMRAI's tests are all in the /source/test directory and are split up by module to match different parts of the codebase. Please review existing tests to determine where your test should go."

Change: Completed the cut-off sentence and corrected the typo "spilt" to "split."