doc/go1.15: replace tab indentation with spaces
doc/go1.15: replace tab indentation with spaces

Replace tab indentation with spaces for consistency, as all other indentation is done with spaces.

For #37419

Change-Id: I728a75ae0d00e637f57eb455b6039ffc1a5feed2
Reviewed-on: https://go-review.googlesource.com/c/go/+/237538
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,akutz/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,akutz/go,golang/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go