refactor: fix spelling in private field names

Fix some misspelled field names.
These field names seem to be used only in private contexts.
Thus, the probability of external code accessing these fields is very
low.
In the rare case of external usage, compile time errors will easily
direct users to the new field name.

tools/pynuttx/nxgdb/stack.py

@@ -111,7 +111,7 @@ class Stack(object):
 
         return self.check_max_usage()
 
-    def avalaible(self):
+    def available(self):
         cur_usage = self.cur_usage()
         return self._stack_size - cur_usage