docs: fix capitalization in doxygen comments

Change-Id: Ie61b20a3dbfc847bc14523e507102fe6fe416908
diff --git a/tests/unit/util/io.t.cpp b/tests/unit/util/io.t.cpp
index 270270c..9fcafd7 100644
--- a/tests/unit/util/io.t.cpp
+++ b/tests/unit/util/io.t.cpp
@@ -114,7 +114,7 @@
   }
 
   /**
-   * \brief create a directory at `filepath`, so that it's neither readable nor writable as a file
+   * \brief Create a directory at `filepath`, so that it's neither readable nor writable as a file.
    */
   void
   mkdir() const