Fix TestAccComputeRoute_hopInstance

[danawillow]

Instance names must start with a letter, RandString doesn't necessarily do that.

$ make testacc TEST=./google TESTARGS='-run=TestAccComputeRoute_hopInstance'==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccComputeRoute_hopInstance -timeout 120m
=== RUN   TestAccComputeRoute_hopInstance
--- PASS: TestAccComputeRoute_hopInstance (72.53s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	72.789s

[ghost]

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