Remove trailing whitespaces (#2806)

While running make notices the following:
```
** presubmit/trailing-whitespace
plugin/grpc/README.md:Multiple upstreams are randomized (see `policy`) on first use. When a proxy returns an error
plugin/pprof/setup_test.go:		{`pprof :1234 {
plugin/pprof/setup_test.go:		{`pprof {
** presubmit/trailing-whitespace: please remove any trailing white space
```

This fix removes the whitespaces
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>