Correct typo for linux-gcc-rel.

When I set up the bot I imagined it was a "buildbot" bot, but AFAIK
it will work fine in LUCI as well. Or actually, I have no idea, but
it appears the chromium.linux buildbot waterfall has gone away.

If there's no change needed on the recipe side, this should be
enough to fix it.

Bug: chromium:776284
Change-Id: I3cff6f866a241db1f4a60446480f6f7c40e95ef7
Reviewed-on: https://chromium-review.googlesource.com/991853Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548001}

infra/config/global/luci-milo-dev.cfg

@@ -403,7 +403,7 @@ consoles: {
     short_name: "tst"
   }
   builders: {
-    name: "buildbot/luci.chromium.ci/linux-gcc-rel"
+    name: "buildbucket/luci.chromium.ci/linux-gcc-rel"
     category: "chromium.linux|release"
     short_name: "gcc"
   }

infra/config/global/luci-milo.cfg

@@ -413,7 +413,7 @@ consoles: {
     short_name: "tst"
   }
   builders: {
-    name: "buildbot/luci.chromium.ci/linux-gcc-rel"
+    name: "buildbucket/luci.chromium.ci/linux-gcc-rel"
     category: "chromium.linux|release"
     short_name: "gcc"
   }