Fix update-colvars-code.sh for Gromacs

grep command used to define COLVARS_VERSION was incorrect after the addition of COLVARS_PATCH_VERSION to colvars_version.h
This id only used to update Gromacs code outside of CI, hence the lack of CI test failure.

Author: jhenin

update-colvars-code.sh

@@ -115,7 +115,7 @@ if [ ${code} == "GROMACS" ] ; then
   fi
 fi
 
-COLVARS_VERSION=$(grep define $(dirname $0)/src/colvars_version.h | cut -d' ' -f 3 | tr -d '"')
+COLVARS_VERSION=$(grep '^#define COLVARS_VERSION' $(dirname $0)/src/colvars_version.h | cut -d' ' -f 3 | tr -d '"')
 if [ -z "${COLVARS_VERSION}" ] ; then
   echo "Error reading Colvars version." >&2
   exit 1