Fix a typo in the comment. (#2866)

I was trying to run this test and I had copied the name of the function
from the comment, and it took a good while to figure out why
`go test -run` was returning `testing: warning: no tests to run`.

test/end2end_test.go

@@ -3535,7 +3535,7 @@ func testMultipleSetHeaderStreamingRPCError(t *testing.T, e env) {
 	}
 }
 
-// TestMalformedHTTP2Metedata verfies the returned error when the client
+// TestMalformedHTTP2Metadata verfies the returned error when the client
 // sends an illegal metadata.
 func (s) TestMalformedHTTP2Metadata(t *testing.T) {
 	for _, e := range listTestEnv() {