tour: Updated description of struct and added type description in moretypes/2

[sagar23sj]

The existing description didnot explain the type keyword and had less explanation of struct. New changes added an explanation on struct and type keyword.

Fixes: golang/tour#121

[gopherbot]

This PR (HEAD: 73c6d3aaf20fae016ce1044ac33b5b96eeffd175) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/328710 to see it.

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