doc: correct typo in PR contribution instructions · nodejs/node@76f21c5

Original file line number	Diff line number	Diff line change
`@@ -187,7 +187,7 @@ A good commit message should describe what changed and why.`
`187`	`187`	`Fixes:` and `Refs:` trailers get automatically added to your commit message
`188`	`188`	`when the Pull Request lands as long as they are included in the`
`189`	`189`	`Pull Request's description. If the Pull Request lands in several commits,`
`190`		`- by default the trailers found in the description are added to each commits.`
	`190`	`+ by default the trailers found in the description are added to each commit.`
`191`	`191`
`192`	`192`	`Examples:`
`193`	`193`