Issue #29109: Enhance tracemalloc documentation

* Wrong parameter name, 'group_by' instead of 'key_type'
* Don't round up numbers when explaining the examples. If they exactly match
  what can be read in the script output, it is to easier to understand
  (4.8 MiB vs 4855 KiB)
* Fix incorrect method link that was pointing to another module

Patch written by Loic Pefferkorn.