docs: Fix link to sigil_H/2

`sigil_H/2` and the HEEx documentation moved from
`Phoenix.LiveView.Helpers` to `Phoenix.Component` in
phoenixframework/phoenix_live_view@02e20e4.

With the 0.18.0 release, this left the HEEx link in the README broken.
This change points to the new URL.

Author: the-mikedavis

README.md

@@ -1,6 +1,6 @@
 # Tree-sitter HEEx
 
-[Tree-sitter](https://tree-sitter.github.io/tree-sitter/) grammar and parser for [HEEx](https://hexdocs.pm/phoenix_live_view/Phoenix.LiveView.Helpers.html#sigil_H/2), the HTML-aware and component-friendly extension of EEx for [Phoenix](https://www.phoenixframework.org/).
+[Tree-sitter](https://tree-sitter.github.io/tree-sitter/) grammar and parser for [HEEx](https://hexdocs.pm/phoenix_live_view/Phoenix.Component.html#sigil_H/2), the HTML-aware and component-friendly extension of EEx for [Phoenix](https://www.phoenixframework.org/).
 
 For EEx support, see [tree-sitter-eex](https://github.com/connorlay/tree-sitter-eex). For Surface support, see [tree-sitter-surface](https://github.com/connorlay/tree-sitter-surface).