fix: update out-of-date comment in exceptions.py (#93)

diff --git a/google/api_core/exceptions.py b/google/api_core/exceptions.py
index a29266d..412fc2e 100644
--- a/google/api_core/exceptions.py
+++ b/google/api_core/exceptions.py
@@ -31,7 +31,7 @@
     grpc = None
 
 # Lookup tables for mapping exceptions from HTTP and gRPC transports.
-# Populated by _APICallErrorMeta
+# Populated by _GoogleAPICallErrorMeta
 _HTTP_CODE_TO_EXCEPTION = {}
 _GRPC_CODE_TO_EXCEPTION = {}