docs: fix typo "foor" -> "foo" in README example (#571)

The JSON example in the stdin section had a typo: "foor" instead of
"foo". Surrounding examples in the same section use "foo" consistently.

Co-authored-by: maxtaran2010 <ocotifuzo727@gmail.com>

README.md

@@ -146,7 +146,7 @@ grpcurl -d @ grpc.server.com:443 my.custom.server.Service/Method <<EOM
 {
   "id": 1234,
   "tags": [
-    "foor",
+    "foo",
     "bar"
   ]
 }