tutorials: fix incipit tutorial

[musicEnfanthen]

This PR adds some improvements for the incipit tutorial, including those proposed in #263 by @mfeustle .

This includes :

• simplifying the note example in step 4;
• simplifying and improving readability of the xml:ids used in the final step;
• improving the explanation of the attributes used in the final step;
• improving the explanation of the referencing mechanism with xml:ids in the final step;
• fixing a typo in the title of the incipit tutorial in the Spanish translation.

Fixes #263

[musicEnfanthen]

@martha-thomae May I ask you to have a look at the updated Spanish translations?

[rettinghaus]

@martha-thomae Could you have a look please?