setvbuf: minor comment typo fix

src/stdio/setvbuf.c

 #include "stdio_impl.h"
 
 /* The behavior of this function is undefined except when it is the first
- * operation on the stream, so the presence or absence of lockign is not
+ * operation on the stream, so the presence or absence of locking is not
  * observable in a program whose behavior is defined. Thus no locking is
  * performed here. No allocation of buffers is performed, but a buffer
  * provided by the caller is used as long as it is suitably sized. */