Update schema-understand.md (#5233)

fix full-width “” to half-width "
diff --git a/site2/docs/schema-understand.md b/site2/docs/schema-understand.md
index f501d66..ff8a973 100644
--- a/site2/docs/schema-understand.md
+++ b/site2/docs/schema-understand.md
@@ -112,10 +112,10 @@
 
 ```text
 {
-    “name”: “test-string-schema”,
-    “type”: “STRING”,
-    “schema”: “”,
-    “properties”: {}
+    "name": "test-string-schema",
+    "type": "STRING",
+    "schema": "",
+    "properties": {}
 }
 ```
 
@@ -252,7 +252,7 @@
         .create();
 
     final int key = 100;
-    final String value = "value-100”;
+    final String value = "value-100";
 
     // send the key/value message
     producer.newMessage()