Fix spelling Attirbute=>Attribute (#128)

* Fix spelling Attirbute=>Attribute

* make error message format closer to other missing attribute error messages in processSingleXMLPluginFile function

Author: josch

pluginlib/include/pluginlib/class_loader_imp.hpp

@@ -668,7 +668,7 @@ void ClassLoader<T>::processSingleXMLPluginFile(
     std::string library_path = library->Attribute("path");
     if (0 == library_path.size()) {
       ROS_ERROR_NAMED("pluginlib.ClassLoader",
-        "Failed to find Path Attirbute in library element in %s", xml_file.c_str());
+        "Attribute 'path' in 'library' tag is missing in %s.", xml_file.c_str());
       continue;
     }