x/website: Minor typo in function comment for SumNumbers()

[baturkey]

What is the URL of the page with the issue?

https://go.dev/doc/tutorial/generics

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/97.0.4692.99 Safari/537.36

Screenshot

What did you do?

What did you expect to see?

// SumNumbers sums the values of map m. It supports both integers
// and floats as map values.

What did you see instead?

// SumNumbers sums the values of map m. Its supports both integers
// and floats as map values.

[gopherbot]

Change https://golang.org/cl/381654 mentions this issue: _content/doc/tutorial: fix a typo