docs: fix capitalization in doxygen comments

Change-Id: Ibf5ee5119d12d60d382b0acef8dfd08277c18fcb
diff --git a/tools/nfdc/help.hpp b/tools/nfdc/help.hpp
index 19df8c1..80b37fe 100644
--- a/tools/nfdc/help.hpp
+++ b/tools/nfdc/help.hpp
@@ -30,7 +30,7 @@
 
 namespace nfd::tools::nfdc {
 
-/** \brief writes the list of available commands to a stream
+/** \brief Writes the list of available commands to a stream.
  *  \param os the output stream to write the list to
  *  \param parser instance of CommandParser containing the commands to list
  *  \param mode only the commands available in this mode are listed
@@ -40,7 +40,7 @@
 helpList(std::ostream& os, const CommandParser& parser,
          ParseMode mode = ParseMode::ONE_SHOT, std::string_view noun = "");
 
-/** \brief tries to help the user, if requested on the command line
+/** \brief Tries to help the user, if requested on the command line.
  *
  *  Depending on the provided command line arguments \p args, this function can either
  *  open the man page for a specific command, or list all commands available in \p parser.