server: use proper Content-Type in curl examples

Without the header Content-Type: application/json, curl will POST with
Content-Type: application/x-www-form-urlencoded

Though our simple server doesn't care, the httplib.h used has a limit
with CPPHTTPLIB_FORM_URL_ENCODED_PAYLOAD_MAX_LENGTH 8192

With Content-Type: application/json, we can send large json data.

examples/server/README.md

@@ -66,6 +66,7 @@ Using [curl](https://curl.se/). On Windows `curl.exe` should be available in the
 ```sh
 curl --request POST \
     --url http://localhost:8080/completion \
+    --header "Content-Type: application/json" \
     --data '{"prompt": "Building a website can be done in 10 simple steps:","n_predict": 128}'
 ```
 

examples/server/chat.sh

@@ -32,6 +32,7 @@ tokenize() {
         --silent \
         --request POST \
         --url "${API_URL}/tokenize" \
+        --header "Content-Type: application/json" \
         --data-raw "$(jq -ns --arg content "$1" '{content:$content}')" \
     | jq '.tokens[]'
 }
@@ -64,6 +65,7 @@ chat_completion() {
         --no-buffer \
         --request POST \
         --url "${API_URL}/completion" \
+        --header "Content-Type: application/json" \
         --data-raw "${DATA}")
 
     printf "\n"