Fix broken links to manual web page in README.md (#5284)

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.

README.md

@@ -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)