Fix Typo

... and this, kids, is why you should always compile in debug before
pushing.

src/core/transport/metadata.c

@@ -484,7 +484,7 @@ void grpc_mdelem_unref(grpc_mdelem *gmd) {
   internal_metadata *md = (internal_metadata *)gmd;
   grpc_mdctx *ctx = md->context;
   lock(ctx);
-  assert(gpr_atm_nobarrier_load(&md->refcnt) >= 1);
+  assert(gpr_atm_no_barrier_load(&md->refcnt) >= 1);
   if (1 == gpr_atm_full_fetch_add(&md->refcnt, -1)) {
     ctx->mdtab_free++;
   }