fix(install): update x64 download link in install script (#1317)

The install script currently attempts to download a file named
docs-builder-linux-amd64.zip, which does not exist. This results in a broken
installation:

	$ curl -sL https://ela.st/docs-builder-install | sh
	Note: Installing to /usr/local/bin requires administrator privileges.
	docs-builder is already installed.
	Running in non-interactive mode. Proceeding with installation...
	Downloading docs-builder for linux/amd64...
	  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
									 Dload  Upload   Total   Spent    Left  Speed
	  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
	100     9  100     9    0     0     17      0 --:--:-- --:--:-- --:--:--     0
	Archive:  docs-builder-linux-amd64.zip
	  End-of-central-directory signature not found.  Either this file is not
	  a zipfile, or it constitutes one disk of a multi-part archive.  In the
	  latter case the central directory and zipfile comment will be found on
	  the last disk(s) of this archive.
	unzip:  cannot find zipfile directory in one of docs-builder-linux-amd64.zip or
			docs-builder-linux-amd64.zip.zip, and cannot find docs-builder-linux-amd64.zip.ZIP, period.
	Error: Failed to extract docs-builder from docs-builder-linux-amd64.zip.

Looking at the releases on github there is no "amd64" release, it was
changed at some point to "x64".

Author: mauri870

install.sh

@@ -7,7 +7,7 @@ ARCH=$(uname -m)
 
 # Map architecture naming
 if [ "$ARCH" = "x86_64" ]; then
-  ARCH="amd64"
+  ARCH="x64"
 elif [ "$ARCH" = "aarch64" ] || [ "$ARCH" = "arm64" ]; then
   ARCH="arm64"
 fi