Fix broken links to manual web page in README.md
The old link to the manual web page includes the `.html` extension but this leads to a 404. Removing the `.html` leads to the right webpage.
@@ -7,12 +7,12 @@ A secure runtime for JavaScript and TypeScript.
Deno aims to provide a productive and secure scripting environment for the
modern programmer. It is built on top of V8, Rust, and TypeScript.
-Please read the [introduction](https://deno.land/manual.html#introduction) for
-more specifics.
+Please read the [introduction](https://deno.land/manual#introduction) for more
+specifics.
[Website](https://deno.land/)
-[Manual](https://deno.land/manual.html)
+[Manual](https://deno.land/manual)
[Install](https://github.com/denoland/deno_install)
