message: doc: Fix name of script guix-build
diff: @@ -131,7 +131,7 @@ export JOBS=1 ADDITIONAL_GUIX_COMMON_FLAGS='--max-jobs=8'
  From the top of a clean Bitcoin Core repository:
  ```sh
  -./contrib/guix/guix-build.sh
  +./contrib/guix/guix-build
  ```
  After the build finishes successfully (check the status code please), compare
  