[mini] Fixing typo in notify finish

[SeverinDiederichs]

There was a typo in notify finish, which led to the warning
mpool.c:43 UCX WARN object 0x11ba3bc0 was not returned to mpool ucp_requests
because the MPI_Wait call for m_psend_request was not executed. Additionally, the buffer m_psend_buffer was not freed. Not freeing the memory would cause the simulation to segfault for larger runs.
This PR fixes the issue and the problem is not occurring anymore.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable