docs: fix undefined tap/1 in getting_started example (closes #46)

[GenericJam]

What

The "Your first screen" example in guides/getting_started.md used:

<Buttontext="Tap me" on_tap={tap(:increment)} />

There is no tap/1 function — use Mob.Screen imports Mob.Sigil (which has no tap helper), so a newcomer pasting the getting-started example hits a compile error: undefined function tap/1. Reported as #46.

Fix

Use the canonical inline tuple, which the example's existing handle_info({:tap, :increment}, ...) already handles:

<Buttontext="Tap me" on_tap={{self(),:increment}} />

This matches the form already used in Mob.Sigil's own docstring (lib/mob/sigil.ex:23), guides/screen_lifecycle.md, and guides/device_capabilities.md — getting_started was the only doc with the broken form. Also added one clause to the walkthrough prose explaining the {self(), :increment} → {:tap, :increment} wiring, since that mapping was previously unexplained.

Verification

Compiled the corrected module (use Mob.Screen + the ~MOB sigil) and called render/1 — it compiles and produces a valid render tree, where the old form was a hard compile error. Docs-only; no code paths touched.

Closes#46.

🤖 Generated with Claude Code