Add erase to example

Use defined blocksize constant

Author: kegilbert

features/filesystem/bd/HeapBlockDevice.h

@@ -34,13 +34,16 @@
  * #include "mbed.h"
  * #include "HeapBlockDevice.h"
  *
- * HeapBlockDevice bd(2048, 512); // 2048 bytes with a block size of 512 bytes
- * uint8_t block[512] = "Hello World!\n";
+ * #define BLOCK_SIZE 512
+ *
+ * HeapBlockDevice bd(2048, BLOCK_SIZE); // 2048 bytes with a block size of 512 bytes
+ * uint8_t block[BLOCK_SIZE] = "Hello World!\n";
  *
  * int main() {
  *     bd.init();
- *     bd.program(block, 0, bd.get_program_size());
- *     bd.read(block, 0, bd.get_read_size());
+ *     bd.erase(0, BLOCK_SIZE);
+ *     bd.program(block, 0, BLOCK_SIZE);
+ *     bd.read(block, 0, BLOCK_SIZE);
  *     printf("%s", block);
  *     bd.deinit();
  * }