Correct instructions for 'make install'

Author: iainrb

README.md

@@ -30,7 +30,7 @@ Installation and testing
 
 * To compile: `make all`
 * To run tests: `make test`. Requires `cxxtestgen` on the PATH. In the WTSI environment, can use `module load cxxtest/$(VERSION)`.
-* To install simtools and normalize_manifest: `make install DIR=$(DESTINATION)` will install executables to the `bin` subdirectory of `$DESTINATION`.
+* To install simtools and normalize_manifest: `make install PREFIX=$(DESTINATION)` will install executables to the `bin` subdirectory of `$DESTINATION`.
 * To install g2i to /software/varinf: `make install_g2i`