fix block_job_set_speed name in documentation

Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hmp-commands.hx

@@ -92,8 +92,8 @@ ETEXI
     },
 
 STEXI
-@item block_job_set_stream
-@findex block_job_set_stream
+@item block_job_set_speed
+@findex block_job_set_speed
 Set maximum speed for a background block operation.
 ETEXI