perf tools: Fix a typo in a variable name in the Documentation Makefile

This patch fix a spelling typo in a variable name in the Documentation Makefile.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190801032812.25018-1-standby24x7@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>