Usage.md: change example code to avoid PHP notice #559

Job	Run time
PHPUnit (PHP 7.2)	35s
Tests alternative autoloader (PHP 7.1)	8s
PHPUnit (PHP 7.3)	38s
Tests alternative autoloader (PHP 7.2)	16s
PHPUnit (PHP 7.4)	22s
Tests alternative autoloader (PHP 7.3)	14s
Tests alternative autoloader (PHP 7.4)	10s
Tests alternative autoloader (PHP 8.0)	10s
PHPUnit (PHP 8.0)	46s
PHPUnit (PHP 8.1)	29s
Tests alternative autoloader (PHP 8.1)	8s
PHPUnit (PHP 8.2)	23s
PHPUnit (PHP 8.3)	54s
Tests alternative autoloader (PHP 8.2)	18s
Tests alternative autoloader (PHP 8.3)	43s
PHPUnit Composer v2 (PHP 7.4)	32s
PHPUnit coverage (PHP 7.4)	44s
PHPUnit (PHP 7.1)	21s
PHPUnit lowest deps (PHP 7.3)	26s
Windows-Tests with PHP 7.1	1m 11s
Windows-Tests with PHP 7.2	41s
Windows-Tests with PHP 7.3	53s
Windows-Tests with PHP 7.4	48s
Windows-Tests with PHP 8.0	1m 0s
Windows-Tests with PHP 8.1	42s
Windows-Tests with PHP 8.2	1m 4s
Windows-Tests with PHP 8.3	58s
	15m 34s

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