trace: Fix typo in allocate_ftrace_ops()'s comment

No functional change, just correct the word.

Link: https://lkml.kernel.org/r/20200610033251.31713-1-richard.weiyang@linux.alibaba.com

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>