fix typo in README.md (#8)

fix typo of "Extended"

Author: tyingq

README.md

@@ -1,6 +1,6 @@
 ![Build Status](https://github.com/WebAssembly/extended-const/actions/workflows/main.yml/badge.svg)
 
-# Extented Const Expressesions Proposal for WebAssembly
+# Extended Const Expressesions Proposal for WebAssembly
 
 This repository is a clone of [github.com/WebAssembly/spec/](https://github.com/WebAssembly/spec/).
 It is meant for discussion, prototype specification and implementation of a proposal to add tail call support to WebAssembly.