PascalCase UndefinedGroupError::getGroupName()

For consistency, as it's only one of two camelCase method names in
the API.

include/loot/exception/undefined_group_error.h

@@ -45,7 +45,7 @@ public:
    * Get the name of the undefined group.
    * @return A group name.
    */
-  std::string getGroupName() { return groupName_; }
+  std::string GetGroupName() { return groupName_; }
 
 private:
   const std::string groupName_;