doc: fix typo in test.md

PR-URL: #47053
Reviewed-By: Moshe Atlow <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Qingyu Deng <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>

Author: vhiairrassary

doc/api/test.md

@@ -549,7 +549,7 @@ to the test runner's output is required, use the events emitted by the
 ### 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}