load_roots: arguments must be files

I had originally thought to support directories with
mbedtls_x509_crt_parse_path but it would have complicated the code more than
I cared for. Remove a remnant of the original project in the documentation.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/x509/load_roots.c b/programs/x509/load_roots.c
index 5baf08b..de9914e 100644
--- a/programs/x509/load_roots.c
+++ b/programs/x509/load_roots.c
@@ -86,7 +86,7 @@
 #define DFL_PRIME_CACHE         1
 
 #define USAGE \
-    "\n usage: load_roots param=<>... [--] {FILE|DIR}...\n"   \
+    "\n usage: load_roots param=<>... [--] FILE...\n"   \
     "\n acceptable parameters:\n"                       \
     "    iterations=%%d        Iteration count (not including cache priming); default: 1\n"  \
     "    prime=%%d             Prime the disk read cache? Default: 1 (yes)\n"  \