runtime/pprof/internal/profile: use idiomatic swapping

gogrep found only one such case with the pattern below:

	$tmp := $x; $x = $y; $y = $tmp

R=1.13

Change-Id: I6e46fb5ef2887f24fa9fc451323a8cef272e2886
Reviewed-on: https://go-review.googlesource.com/c/151200
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/pprof/internal/profile/profile.go

@@ -211,9 +211,7 @@ func (p *Profile) setMain() {
 			continue
 		}
 		// Swap what we guess is main to position 0.
-		tmp := p.Mapping[i]
-		p.Mapping[i] = p.Mapping[0]
-		p.Mapping[0] = tmp
+		p.Mapping[i], p.Mapping[0] = p.Mapping[0], p.Mapping[i]
 		break
 	}
 }