_content/doc/tutorial: fix a typo

Fixes golang/go#50853

Change-Id: Ia09043435f23b979a2a0d198318096e6ecf52cb5
Reviewed-on: https://go-review.googlesource.com/c/website/+/381654
Reviewed-by: DO NOT USE <iant@google.com>
Trust: Cherry Mui <cherryyz@google.com>

_content/doc/tutorial/generics.md

@@ -417,7 +417,7 @@ section.
     `SumNumbers` function.
 
     ```
-    // SumNumbers sums the values of map m. Its supports both integers
+    // SumNumbers sums the values of map m. It supports both integers
     // and floats as map values.
     func SumNumbers[K comparable, V Number](m map[K]V) V {
         var s V