docs: Update the security e-mail address. #1644

Job	Run time
Tests (ubuntu-20.04, 3.8, 16, django32-drflatest)	1m 32s
Tests (ubuntu-20.04, 3.8, 16, django40-drflatest)	1m 56s
Tests (ubuntu-20.04, 3.8, 16, django42-drflatest)	1m 35s
Tests (ubuntu-20.04, 3.8, 16, quality)	1m 48s
Tests (ubuntu-20.04, 3.8, 16, pii_check)	1m 3s
Tests (ubuntu-20.04, 3.8, 16, version_check)	48s
Tests (ubuntu-20.04, 3.8, 16, js_tests)	1m 23s
Tests (ubuntu-20.04, 3.8, 16, js_lint)	1m 23s
Tests (ubuntu-20.04, 3.8, 16, rst_validation)	46s
Tests (ubuntu-20.04, 3.8, 16, translations-django32)	1m 22s
Tests (ubuntu-20.04, 3.8, 16, translations-django42)	1m 35s
	15m 11s

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