`custom_environment.index.md`: typo fix parallel API link #1038

Job	Run time
tutorial-test (3.8, Tianshou)	3m 19s
tutorial-test (3.8, CustomEnvironment)	29s
tutorial-test (3.8, CleanRL)	5m 31s
tutorial-test (3.8, SB3/kaz)	5m 2s
tutorial-test (3.8, SB3/waterworld)	8m 43s
tutorial-test (3.8, SB3/connect_four)	3m 6s
tutorial-test (3.8, SB3/test)	2m 13s
tutorial-test (3.8, AgileRL)	14m 6s
tutorial-test (3.9, Tianshou)	3m 23s
tutorial-test (3.9, CustomEnvironment)	32s
tutorial-test (3.9, CleanRL)	6m 10s
tutorial-test (3.9, SB3/kaz)	4m 56s
tutorial-test (3.9, SB3/waterworld)	7m 53s
tutorial-test (3.9, SB3/connect_four)	3m 23s
tutorial-test (3.9, SB3/test)	2m 12s
tutorial-test (3.9, AgileRL)	14m 54s
tutorial-test (3.10, Tianshou)	3m 25s
tutorial-test (3.10, CustomEnvironment)	38s
tutorial-test (3.10, CleanRL)	7m 38s
tutorial-test (3.10, SB3/kaz)	5m 2s
tutorial-test (3.10, SB3/waterworld)	8m 8s
tutorial-test (3.10, SB3/connect_four)	2m 54s
tutorial-test (3.10, SB3/test)	2m 7s
tutorial-test (3.10, AgileRL)	15m 19s
tutorial-test (3.11, Tianshou)	3m 43s
tutorial-test (3.11, CustomEnvironment)	30s
tutorial-test (3.11, CleanRL)	7m 4s
tutorial-test (3.11, SB3/kaz)	4m 39s
tutorial-test (3.11, SB3/waterworld)	8m 2s
tutorial-test (3.11, SB3/connect_four)	3m 0s
tutorial-test (3.11, SB3/test)	2m 4s
tutorial-test (3.11, AgileRL)	17m 57s
	2h 58m 2s

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