tracing/histogram: Update document for KEYS_MAX size

After commit 4f36c2d ("tracing: Increase tracing map KEYS_MAX size"),
'keys' supports up to three fields.

Signed-off-by: Zheng Yejian <[email protected]>
Cc: [email protected]
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: Zheng Yejian

Documentation/trace/histogram.rst

@@ -39,7 +39,7 @@ Documentation written by Tom Zanussi
   will use the event's kernel stacktrace as the key.  The keywords
   'keys' or 'key' can be used to specify keys, and the keywords
   'values', 'vals', or 'val' can be used to specify values.  Compound
-  keys consisting of up to two fields can be specified by the 'keys'
+  keys consisting of up to three fields can be specified by the 'keys'
   keyword.  Hashing a compound key produces a unique entry in the
   table for each unique combination of component keys, and can be
   useful for providing more fine-grained summaries of event data.