Fix nginx to use correct url (#5720)

frameworks/C/nginx/benchmark_config.json

@@ -3,7 +3,7 @@
   "tests": [
     {
       "default": {
-        "plaintext_url": "/",
+        "plaintext_url": "/plaintext",
         "port": 8080,
         "approach": "Realistic",
         "classification": "Platform",

frameworks/C/nginx/nginx.conf

@@ -29,7 +29,7 @@ http {
                     #8080 default_server reuseport deferred backlog=65535 fastopen=4096;
         root /;
         
-        location = / {
+        location = /plaintext {
             default_type text/plain;
             return 200 "Hello, World!";
         }