Fix sed error in podspecs on sed 4.4
diff --git a/templates/gRPC-Core.podspec.template b/templates/gRPC-Core.podspec.template
index 9785d15..da404e2 100644
--- a/templates/gRPC-Core.podspec.template
+++ b/templates/gRPC-Core.podspec.template
@@ -202,6 +202,6 @@
 
     # TODO (mxyan): Instead of this hack, add include path "third_party" to C core's include path?
     s.prepare_command = <<-END_OF_COMMAND
-      find src/core/ -type f -exec sed -E -i '.back' 's;#include "third_party/nanopb/(.*)";#include <nanopb/\\1>;g' {} \\\;
+      find src/core/ -type f -exec sed -E -i'.back' 's;#include "third_party/nanopb/(.*)";#include <nanopb/\\1>;g' {} \\\;
     END_OF_COMMAND
   end