Fix #167: unmatched quotes in compilation script

Remove back quotes from the usage message, which were interpreted as
command substitution by the shell and caused the script to fail. Also
update the usage message and fix a couple typos.

HEN_HOUSE/scripts/compile_user_code

@@ -40,18 +40,14 @@ if test $# -lt 1; then
  cat >&2 <<EOF
 Usage: $my_name m|f|mf user_code [a] [opt|noopt|debug] [config=xxx]
 
-$my_name compiles the EGSnrc user code 'user_code'.
-The arguments `f' and `mf' are equivalent, `m' results in a
-mortan compilation only.
-The argument 'a' is here for historical reasons to provide
-compatibility with the typical invocation of EGS compilations in
-previous versions (e.g. 'mf user_code a opt2'). It is ignored if present.
-The optional argumens 'opt' or 'noopt' or 'debug' set optimization
-or no optimization or debug compilation flags for the Fortran compiler.
-'opt' is the default. The 'config=xxx' option lets you overwrite
-the EGSnrc configuration to be used, which is otherwise specified by
-the EGS_CONFIG environment variable. Note that 'xxx' MUST
-be an absolute path to a EGSnrc configuration file.
+$my_name compiles the EGSnrc user code 'user_code'. The arguments 'f' and 'mf'
+are equivalent: 'm' results in a mortan compilation only. The argument 'a' is
+here for historical reasons and is now ignored. The optional arguments 'opt' or
+'noopt' or 'debug' respectively sets optimization, turns off optimization or
+sets the debug compilation flags for the Fortran compiler ('opt' is the
+default). The 'config=xxx' option lets you overwrite the EGSnrc configuration
+to be used, which is otherwise specified by the EGS_CONFIG environment variable.
+Note that 'xxx' MUST be an absolute path to a EGSnrc configuration file.
 
 EOF
  exit 1