Fix grammar

Author: fthomas

repos.md

@@ -1,6 +1,6 @@
 List the repositories here you want Scala Steward to keep up-to-date.
 The format is "- $owner/$repo".
-If you want Scala Steward to keep a non-default branch up-to-date use "- $owner/$repo:$branch".
+If you want Scala Steward to keep a non-default branch up-to-date, use "- $owner/$repo:$branch".
 All lines that do not start with a hyphen and space are ignored.
 
 - scala-steward-org/scala-steward