_content/doc: fix parseVersionNumber to handle versions without patch

As of go1.20, when visiting https://go.dev/doc/install, the download button shows no version and with text: `Download ()`.

Previously the function expected the argument `string` being passed as in the form: `go1.17.3.linux-amd64.tar.gz`.
with go1.20 the file name becomes `go1.20.linux-amd64.tar.gz` with no patch number.

This merge request fix the regex used to parse the version number and return the intended version of `1.20`

Change-Id: I2fd109a030ed940cd8cd4d5e8924f026f5b8f4be
GitHub-Last-Rev: 68fb7856ee
GitHub-Pull-Request: golang/website#192
Reviewed-on: https://go-review.googlesource.com/c/website/+/465415
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

_content/doc/download.js

@@ -131,8 +131,8 @@ class DownloadsController {
 
   // get version number.
   parseVersionNumber(string) {
-    const rx = /(\d+\.)(\d+\.)(\d+)/g;
-    const matches = rx.exec(string)
+    const rx = /(\d+\.)(\d+)(\.\d+)?/g;
+    const matches = rx.exec(string);
     if (matches?.[0]) {
       return matches[0];
     } else {