DOC: cache: Use '<name>' instead of '<id>' in error message

When the cache name is left out in 'filter cache' the error message refers
to a missing '<id>'. The name of the cache is called 'name' within the docs.

Adjust the error message for consistency.

The error message was introduced in 99a17a2d91f9044ea20bba6617048488aed80555.
This commit first appeared in 1.9, thus the patch must be backported to 1.9+.
diff --git a/src/cache.c b/src/cache.c
index 533b902..4f8d2e0 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1407,7 +1407,7 @@
 
 	/* Get the cache filter name. <pos> point on "cache" keyword */
 	if (!*args[pos + 1]) {
-		memprintf(err, "%s : expects an <id> argument", args[pos]);
+		memprintf(err, "%s : expects a <name> argument", args[pos]);
 		goto error;
 	}
 	name = strdup(args[pos + 1]);