ccp4 README: fix modbuild flags per 2.1.3 --help

No --check-mode exists; --prep is the real way to pre-fetch and check the
tmp_dir config. modbuild on Ra is 2.1.3 now.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ccp4/README.md

@@ -31,9 +31,9 @@ EOF
 Then:
 
 ```bash
-module use unstable && module load modbuild/2.1.2
+module use unstable && module load modbuild/2.1.3
 cd ccp4
-modbuild build --check-mode 9.0     # yamllint the config first
+modbuild build --prep 9.0   # download/cache only; also verifies tmp_dir took effect
 modbuild build 9.0
 ```