gdb: fix map prettyprinter

(gdb) p x
Python Exception <class 'gdb.error'> There is no member named b.:
$2 = map[string]string
->
(gdb) p x
$1 = map[string]string = {["shane"] = "hansen"}

Change-Id: I874d02a029f2ac9afc5ab666afb65760ec2c3177
Reviewed-on: https://go-review.googlesource.com/5522
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/runtime/runtime-gdb.py b/src/runtime/runtime-gdb.py
index 47a28f5..9f2ba9f 100644
--- a/src/runtime/runtime-gdb.py
+++ b/src/runtime/runtime-gdb.py
@@ -114,7 +114,7 @@
 		return str(self.val.type)
 
 	def children(self):
-		B = self.val['b']
+		B = self.val['B']
 		buckets = self.val['buckets']
 		oldbuckets = self.val['oldbuckets']
 		flags = self.val['flags']