Update network_peering.html.markdown

[themantissa]

remove un-needed parameter from the GCP example - checked at https://docs.atlas.mongodb.com/reference/api/vpc-create-peering-connection/ and https://github.com/terraform-providers/terraform-provider-mongodbatlas/blob/master/mongodbatlas/resource_mongodbatlas_network_peering_test.go

[themantissa]

@PacoDw can you check this? I think there was an extra variable not needed in the GCP example so removed. Thanks!

[PacoDw]

That's correct, thank you so much :)