runtime: fix string formatting

Change-Id: I87d0bc78a246e479d97b3f83cf77c1f701975413
GitHub-Last-Rev: 22cd684e08464f0e01f1cba2235443371dba3a5d
GitHub-Pull-Request: golang/go#29157
Reviewed-on: https://go-review.googlesource.com/c/153298
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/runtime-gdb.py

@@ -353,7 +353,8 @@ class IfacePrinter:
 			return "<bad dynamic type>"
 
 		if dtype is None:  # trouble looking up, print something reasonable
-			return "({0}){0}".format(iface_dtype_name(self.val), self.val['data'])
+			return "({typename}){data}".format(
+				typename=iface_dtype_name(self.val), data=self.val['data'])
 
 		try:
 			return self.val['data'].cast(dtype).dereference()