testing: fix reference to B.N in docstring

Currently, a documentation reference to a struct member (such as [B.N])
does not result in it being rendered as a link, and thus the square
brackets remain in the rendered documentation which is mildly confusing.

The issue can be seen at
https://pkg.go.dev/testing@master#hdr-b_N_style_benchmarks

Remove the square brackets to fix.

Change-Id: Id374fd2085bd511018220c5d663650f89672302e
Reviewed-on: https://go-review.googlesource.com/c/go/+/643496
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/testing/testing.go

@@ -120,7 +120,7 @@
 // # b.N-style benchmarks
 //
 // Prior to the introduction of [B.Loop], benchmarks were written in a
-// different style using [B.N]. For example:
+// different style using B.N. For example:
 //
 //	func BenchmarkRandInt(b *testing.B) {
 //	    for range b.N {