perf hists browser: Use '/' for search/filter instead of 's'

That is what is used in vi and mutt, and as well on the 'annotate'
browser.

Eventually we can have keymappings to make people used to other key
associations more confortable.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
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-fyln9286b8gx5q4n277l0djs@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>