Fix mnesia doc after new doc system #15778

Job	Run time
Event File	4s
Build Erlang/OTP (64-bit)	5m 53s
Build Erlang/OTP (iOS)	9m 5s
Build Erlang/OTP (Windows)	14m 4s
Build Erlang/OTP (Types and Flavors)	9m 18s
Build Erlang/OTP (32-bit)	10m 3s
Build Erlang/OTP (macOS)	6m 31s
Build Erlang/OTP (cross-compile)	15m 48s
Build Erlang/OTP (clang)	6m 33s
Build and check documentation	5m 1s
Run static analysis	6m 20s
Test Erlang/OTP (mnesia)	19m 32s
Release Erlang/OTP	0s
Test Erlang/OTP (system)	8m 18s
	1h 56m 30s

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