edits+readability: rs-add-arbiter

Old
---
file: build/rs-arbiter/json/tutorial/add-replica-set-arbiter.json
source: source/tutorial/add-replica-set-arbiter.txt
stats:
   coleman-liau: 10.437106382978723
   flesch-ease: 40.07791666666668
   flesch-level: 12.5343085106383
   foggy:
      count: 11
      factor: 0.04680851063829787
      threshold: 3
   sentence-count: 12
   sentence-len-avg: 19
   smog-index: 8.59863814320734
   word-count: 235
New
---
file: build/rs-arbiter/json/tutorial/add-replica-set-arbiter.json
source: source/tutorial/add-replica-set-arbiter.txt
stats:
   coleman-liau: 9.850819672131145
   flesch-ease: 44.21676470588238
   flesch-level: 10.657647058823532
   foggy:
      count: 14
      factor: 0.05737704918032787
      threshold: 3
   sentence-count: 17
   sentence-len-avg: 14
   smog-index: 8.313332769828598
   word-count: 244

Signed-off-by: Sam Kleinman <samk@10gen.com>
edits: rs-add-arb
minor: changing introduction title