[string] update doxygen api closing comment (#6331)

src/core/common/string.hpp

@@ -263,11 +263,6 @@ private:
     char     mBuffer[kSize];
 };
 
-/**
- * @}
- *
- */
-
 /**
  * This function validates whether a given byte sequence (string) follows UTF-8 encoding.
  *
@@ -291,6 +286,11 @@ bool IsValidUtf8String(const char *aString);
  */
 bool IsValidUtf8String(const char *aString, size_t aLength);
 
+/**
+ * @}
+ *
+ */
+
 } // namespace ot
 
 #endif // STRING_HPP_