cros_gdb: fix format string for missing sysroot error

Python 2 would ignore extra %s in a format string, whereas Python 3
throws a TypeError. The author clearly just forgot the parenthesis
here, let's add them.

BUG=chromium:1029429,chromium:997354
TEST=CQ

Change-Id: I998f0e79d19d23434848ed93ceccdd9b848d0d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1946442
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
diff --git a/scripts/cros_gdb.py b/scripts/cros_gdb.py
index e682913..7ec82ed 100644
--- a/scripts/cros_gdb.py
+++ b/scripts/cros_gdb.py
@@ -142,8 +142,8 @@
               '+' + 'sysroot_chromeos-base_chromeos-chrome.tar.xz'
     sysroot = os.path.join(self.sdk_path, sysroot)
     if not os.path.isdir(sysroot):
-      raise GdbMissingSysrootError('Cannot find sysroot for %s at.'
-                                   ' %s' % self.board, sysroot)
+      raise GdbMissingSysrootError('Cannot find sysroot for %s at %s'
+                                   % (self.board, sysroot))
     return sysroot
 
   def GetSimpleChromeBinary(self):