doc: fix inconsistency in documentation for building

Specifically, fix the inconsistency where the documentation
suggests running "$ make test" instead of "$ make -j4 test".
The "-j4" flag uses multiple processes, making the command
faster.

Fixes: #19919

PR-URL: #20091
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: spwg

BUILDING.md

@@ -179,10 +179,10 @@ If you are running tests prior to submitting a Pull Request, the recommended
 command is:
 
 ```console
-$ make test
+$ make -j4 test
 ```
 
-`make test` does a full check on the codebase, including running linters and
+`make -j4 test` does a full check on the codebase, including running linters and
 documentation tests.
 
 Optionally, continue below.