Fix d8 prompt after readline removal

TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1246983004 .

Cr-Commit-Position: refs/heads/master@{#29824}

src/d8.cc

@@ -1391,7 +1391,7 @@ void Shell::RunShell(Isolate* isolate) {
   printf("V8 version %s\n", V8::GetVersion());
   while (true) {
     HandleScope inner_scope(isolate);
-    printf(" d8>");
+    printf("d8> ");
 #if defined(__native_client__)
     // Native Client libc is used to being embedded in Chrome and
     // has trouble recognizing when to flush.