testing: fix typo

Fixes #10794.

Change-Id: Id91485394ddbadc28c800e1d0c3ec281ba6cd098
Reviewed-on: https://go-review.googlesource.com/9990Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -44,7 +44,7 @@
 //     }
 //
 // The benchmark function must run the target code b.N times.
-// During benchark execution, b.N is adjusted until the benchmark function lasts
+// During benchmark execution, b.N is adjusted until the benchmark function lasts
 // long enough to be timed reliably.  The output
 //     BenchmarkHello    10000000    282 ns/op
 // means that the loop ran 10000000 times at a speed of 282 ns per loop.