docs: Fix a few typos

There are small typos in:
- CONTRIBUTING.md
- guides/README.md
- src/nodes/midi.js

Fixes:
- Should read `syntax` rather than `sintax`.
- Should read `outputs` rather than `otputs`.
- Should read `dragging` rather than `draging`.
- Should read `build` rather than `bulid`.
- Should read `assume` rather than `asume`.

src/nodes/midi.js

@@ -196,7 +196,7 @@
                 return MIDIEvent.TIMETICK;
                 break;
             default:
-                return Number(str); //asume its a hex code
+                return Number(str); //assume its a hex code
         }
     };