doc: fix typo in test.md

PR-URL: #47053
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Qingyu Deng <i@ayase-lab.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: vhiairrassary

doc/api/test.md

@@ -532,7 +532,7 @@ The following built-reporters are supported:
 ### Custom reporters
 
 [`--test-reporter`][] can be used to specify a path to custom reporter.
-a custom reporter is a module that exports a value
+A custom reporter is a module that exports a value
 accepted by [stream.compose][].
 Reporters should transform events emitted by a {TestsStream}