Update the SDK URL in the README.

Signed-off-by: Wren Turkal <wt@penguintechs.org>

README.md

@@ -4,7 +4,7 @@
 
 This is the OpenWrt "packages"-feed containing community-maintained build scripts, options and patches for applications, modules and libraries used within OpenWrt.
 
-Installation of pre-built packages is handled directly by the **opkg** utility within your running OpenWrt system or by using the [OpenWrt SDK](https://openwrt.org/docs/guide-developer/obtain.firmware.sdk) on a build system.
+Installation of pre-built packages is handled directly by the **opkg** utility within your running OpenWrt system or by using the [OpenWrt SDK](https://openwrt.org/docs/guide-developer/using_the_sdk) on a build system.
 
 ## Usage