Update --help message in db_bench.

Summary:
Remove --help entry for readhot.

Update read_random_exp_range flag description: The distribution is num *
exp(-r), not num * exp(r).

Test Plan: Run ./db_bench --help

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D42303

db/db_bench.cc

@@ -130,8 +130,6 @@ DEFINE_string(benchmarks,
               "\treadreverse   -- read N times in reverse order\n"
               "\treadrandom    -- read N times in random order\n"
               "\treadmissing   -- read N missing keys in random order\n"
-              "\treadhot       -- read N times in random order from 1% section "
-              "of DB\n"
               "\treadwhilewriting      -- 1 writer, N threads doing random "
               "reads\n"
               "\treadwhilemerging      -- 1 merger, N threads doing random "
@@ -232,7 +230,7 @@ DEFINE_double(compression_ratio, 0.5, "Arrange to generate values that shrink"
 
 DEFINE_double(read_random_exp_range, 0.0,
               "Read random's key will be generated using distribution of "
-              "num * exp(r) where r is uniform number from 0 to this value. "
+              "num * exp(-r) where r is uniform number from 0 to this value. "
               "The larger the number is, the more skewed the reads are. "
               "Only used in readrandom and multireadrandom benchmarks.");