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.comReviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 883b4aee ("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>

samples/trace_events/trace-events-sample.h

@@ -155,7 +155,7 @@
  *
  *         To assign this string, use the helper macro __assign_str_len().
  *
- *         __assign_str(foo, bar, len);
+ *         __assign_str_len(foo, bar, len);
  *
  *         Then len + 1 is allocated to the ring buffer, and a nul terminating
  *         byte is added. This is similar to: