doc: fix overriding of prefix option

Make the example in the "Building a debug build" section work as
intended.

Fixes: nodejs#30477

PR-URL: nodejs#30518
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

BUILDING.md

@@ -476,7 +476,7 @@ To use the debug build with all the normal dependencies overwrite the release
 version in the install directory:
 
 ``` console
-$ make install --prefix=/opt/node-debug/
+$ make install PREFIX=/opt/node-debug/
 $ cp -a -f out/Debug/node /opt/node-debug/node
 ```