Change 'Documents' from plural to singular in DocEvent

[chacha912]

What this PR does / why we need it:

Change "documents" to "document" in DocEvent

Which issue(s) this PR fixes:

Fixes #612

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #613 (9953d5a) into main (932a9c7) will not change coverage.
The diff coverage is 11.11%.

@@           Coverage Diff           @@
##             main     #613   +/-   ##
=======================================
  Coverage   50.24%   50.24%           
=======================================
  Files          69       69           
  Lines        7314     7314           
=======================================
  Hits         3675     3675           
  Misses       3147     3147           
  Partials      492      492           

Files Changed	Coverage Δ
api/converter/from_pb.go	47.77% <0.00%> (ø)
api/converter/to_pb.go	55.07% <0.00%> (ø)
client/client.go	13.48% <0.00%> (ø)
pkg/document/internal_document.go	13.67% <ø> (ø)
server/rpc/admin_server.go	53.97% <0.00%> (ø)
server/rpc/yorkie_server.go	56.82% <100.00%> (ø)

[hackerwins]

LGTM.