Fix a typo in use of a configuration setting

nuttx/mm/mm_sbrk.c

@@ -73,8 +73,10 @@
  *    program break is returned by sbrk(0). 
  *
  * Input Parameters:
+ *    heap - A reference to the data structure that defines this heap.
  *    incr - Specifies the number of bytes to add or to remove from the
  *      space allocated for the process.
+      maxbreak - The maximum permissible break address.
  *
  * Returned Value:
  *    Upon successful completion, sbrk() returns the prior break value.

nuttx/mm/umm_sbrk.c

@@ -107,7 +107,7 @@
 
 FAR void *sbrk(intptr_t incr)
 {
-  return mm_sbrk(USR_HEAP, incr, CONFIG_ARCH_STACK_NPAGES << MM_PGSHIFT);
+  return mm_sbrk(USR_HEAP, incr, CONFIG_ARCH_HEAP_NPAGES << MM_PGSHIFT);
 }
 
 #endif /* CONFIG_ARCH_ADDRENV && CONFIG_MM_PGALLOC && ... */