[docs] Fixing rule name in coverage.md docs (#1162)

The docs explains that for activating coverage support we use
`register_coverage_tool = True` inside the rule
`register_python_toolchains`. There is no such rule, the actual rule
name is `python_register_toolchains`

Signed-off-by: Andrés Felipe Barco Santa <andres@engflow.com>

Author: anfelbar