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>

plugin/grpc/README.md

@@ -22,7 +22,7 @@ grpc FROM TO...
 * **TO...** are the destination endpoints to proxy to. The number of upstreams is
   limited to 15.
 
-Multiple upstreams are randomized (see `policy`) on first use. When a proxy returns an error 
+Multiple upstreams are randomized (see `policy`) on first use. When a proxy returns an error
 the next upstream in the list is tried.
 
 Extra knobs are available with an expanded syntax:

plugin/pprof/setup_test.go

@@ -23,10 +23,10 @@ func TestPProf(t *testing.T) {
         }`, true},
 		{`pprof { block
                 }`, false},
-		{`pprof :1234 { 
+		{`pprof :1234 {
                    block 20
                 }`, false},
-		{`pprof { 
+		{`pprof {
                    block 20 30
                 }`, true},
 		{`pprof