tour: mention generic functions in generics/1

[crisman]

The first slide of generics (about generic functions) never uses the phrase "generic function". Adding to the confusion, the next slide (generics/2) then says "In addition to generic functions..." as if the reader should know about them.

Added the name "generic function" to generics/1 to explain that adding type parameters to a func make a generic function.

[gopherbot]

This PR (HEAD: 9f26d78c8839dde6bf8389cd09a0f01c4c0d987f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/471655 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info