doc: fix typo in `test/common/README.md`

doc: fix typo in `test/common/README.md` · nodejs/node@cff3725

Original file line number	Diff line number	Diff line change
`@@ -14,7 +14,7 @@ several other tasks:`
`14`	`14`	`don't accidentally pollute the global namespace.`
`15`	`15`
`16`	`16`	* Some tests assume a default umask of `0o022`. To enforce this assumption,
`17`		`- the common module sets the unmask at startup. Tests that require a`
	`17`	`+ the common module sets the umask at startup. Tests that require a`
`18`	`18`	`different umask can override this setting after loading the module.`
`19`	`19`
`20`	`20`	* Some tests specify runtime flags (example, `--expose-internals`) via a