update polymer init command

[AlrikOlson]

Changed template name value to "custom-build" from "polymer-starter-kit-custom-build"

When running the old suggested command, the result was:

"Template polymer-init-polymer-starter-kit-custom-build:app not found"

Updating to "custom-build" fixed this issue.

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[AlrikOlson]

I signed it!

[googlebot]

CLAs look good, thanks!

[abdonrd]

Thanks @AlrikOlson!

A bit discussion first: #84.