perf annotate browser: Improve description of '?' hotkey

The previous description: "Search previous string" is usually associated
with the 'N' following a '/string', the opposite of 'n', which is
'Search next string' in the direction established with '/' or '?'.

So change it to 'Search string backwards', to clarify that.

The 'N' hotkey remains to be implemented with the semantic described
above.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-5lw5y15d7vv308xbpm8pqe4g@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/browsers/annotate.c

@@ -732,7 +732,7 @@ static int annotate_browser__run(struct annotate_browser *browser,
 		"s             Toggle source code view\n"
 		"/             Search string\n"
 		"r             Run available scripts\n"
-		"?             Search previous string\n");
+		"?             Search string backwards\n");
 			continue;
 		case 'r':
 			{