[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>
diff --git a/docs/coverage.md b/docs/coverage.md
index bc613f8..63f2578 100644
--- a/docs/coverage.md
+++ b/docs/coverage.md
@@ -23,7 +23,7 @@
 For WORKSPACE configuration:
 
 ```starlark
-register_python_toolchains(
+python_register_toolchains(
    register_coverage_tool = True,
 )
 ```