net/rpc: fix args order in strings.Contains call

The old code looks suspicious and is fragile.
It would fail if error messages were not totally the same.
Swapped the arguments order to fix that.

Change-Id: Id5df7242fb9224d0090245286ef8986ebb15e921
Reviewed-on: https://go-review.googlesource.com/c/161157
Run-TryBot: Iskander Sharipov <quasilyte@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/net/rpc/client_test.go

@@ -57,7 +57,7 @@ func TestGobError(t *testing.T) {
 		if err == nil {
 			t.Fatal("no error")
 		}
-		if !strings.Contains("reading body EOF", err.(error).Error()) {
+		if !strings.Contains(err.(error).Error(), "reading body EOF") {
 			t.Fatal("expected `reading body EOF', got", err)
 		}
 	}()