Update 042-string-intro.md

Fix the size of array.

042-string-intro.md

@@ -414,7 +414,7 @@ char32_t s3[] = U"𦥑" ;
 
 ~~~cpp
 char8_t s1[5] = { 0xf0, 0xa6, 0xa5, 0x91, 0x0 } ;
-char16_t s2[2] = { 0xd85a, 0xdd51, 0x0 } ;
+char16_t s2[3] = { 0xd85a, 0xdd51, 0x0 } ;
 char32_t s3[2] = { 0x26951, 0x0 } ;
 ~~~