tracing: Fix mismatched comment in __string_len

tracing: Fix mismatched comment in __string_len

Here __assign_str_len() should be used for the __string_len type, instead
of __assign_str() in the comment.

Link: https://lkml.kernel.org/r/5c012db463392d0e6d4f0636203d778962ad060a.1640170494.git.geliang.tang@suse.com
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 522f2b34fcdcc ("tracing: Add trace_event helper macros __string_len() and __assign_str_len()")
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>