remove hyphen from named extra in api_core (#6468)

diff --git a/setup.py b/setup.py
index 46f6de0..5a7fe04 100644
--- a/setup.py
+++ b/setup.py
@@ -40,7 +40,7 @@
 ]
 extras = {
     'grpc': 'grpcio >= 1.8.2',
-    'grpcio-gcp': 'grpcio-gcp >= 0.2.2'
+    'grpcgcp': 'grpcio-gcp >= 0.2.2'
 }