Tiny grammatical fix in tutorial.txt.

Author: garyscavone

doc/doxygen/tutorial.txt

@@ -4,7 +4,7 @@
 
 \section intro Introduction
 
-RtMidi is a set of C++ classes (RtMidiIn, RtMidiOut and API-specific classes) that provides a common API (Application Programming Interface) for realtime MIDI input/output across Linux (ALSA & JACK), Macintosh OS X (CoreMIDI & JACK), and Windows (Multimedia Library & UWP), Web MIDI, iOS and Android systems.  RtMidi significantly simplifies the process of interacting with computer MIDI hardware and software.  It was designed with the following goals:
+RtMidi is a set of C++ classes (RtMidiIn, RtMidiOut and API-specific classes) that provides a common API (Application Programming Interface) for realtime MIDI input/output across Linux (ALSA & JACK), Macintosh OS X (CoreMIDI & JACK), Windows (Multimedia Library & UWP), Web MIDI, iOS and Android systems.  RtMidi significantly simplifies the process of interacting with computer MIDI hardware and software.  It was designed with the following goals:
 
 - object oriented C++ design
 - simple, common API across all supported platforms