Increase buffer size.

The smaller size is a warning/error on some targets:

```
error: ‘ bytes’ directive output may be truncated writing 6 bytes into a region of size between 0 and 15
```
PiperOrigin-RevId: 823047552
Change-Id: Id66b5828cf84a512dffc452753f1881deb685584

src/engine/engine_print.c

@@ -344,7 +344,7 @@ static void printVector(const char* str, const mjtNum* data, int n, FILE* fp,
 
 // print human readable memory size
 static const char* memorySize(size_t nbytes) {
-  static mjTHREADLOCAL char message[20];
+  static mjTHREADLOCAL char message[32];
   int k = 1024;
 
   if (nbytes < k) {