doc: Clarify doxygen in GroupManager and GroupManagerDB

Change-Id: I35404474de4943cd8c45fecf78cb9a1d335f4e06
Refs: #3287, #3288, #3297
diff --git a/src/group-manager.cpp b/src/group-manager.cpp
index 345eb79..4bbf19e 100644
--- a/src/group-manager.cpp
+++ b/src/group-manager.cpp
@@ -28,10 +28,10 @@
 namespace ndn {
 namespace gep {
 
-GroupManager::GroupManager(const Name& prefix, const Name& dataType, const std::string& dbDir,
+GroupManager::GroupManager(const Name& prefix, const Name& dataType, const std::string& dbPath,
                            const int paramLength, const int freshPeriod)
   : m_namespace(prefix)
-  , m_db(dbDir)
+  , m_db(dbPath)
   , m_paramLength(paramLength)
   , m_freshPeriod(freshPeriod)
 {