[docs] Fix references to non-existent methods

[kevburnsjr]

Fixes references to non-existent methods in documentation (no code changes)

• StartShard -> StartReplica
• StartOnDiskShard -> StartOnDiskReplica
• StartConcurrentShard -> StartConcurrentReplica
• RequestAddNode -> RequestAddReplica
• RequestRemoveNode -> RequestDeleteReplica

[codecov-commenter]

Codecov Report

Base: 84.37% // Head: 84.45% // Increases project coverage by +0.08% :tada:

Coverage data is based on head (8f5ecf1) compared to base (aadd6a9). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #258 +/- ## ========================================== + Coverage 84.37% 84.45% +0.08% ========================================== Files 10 10 Lines 4261 4261 ========================================== + Hits 3595 3598 +3 + Misses 406 403 -3 Partials 260 260 ``` | [Impacted Files](https://codecov.io/gh/lni/dragonboat/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni) | Coverage Δ | | |---|---|---| | [nodehost.go](https://codecov.io/gh/lni/dragonboat/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni#diff-bm9kZWhvc3QuZ28=) | `86.26% <ø> (ø)` | | | [node.go](https://codecov.io/gh/lni/dragonboat/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni#diff-bm9kZS5nbw==) | `79.80% <0.00%> (+0.10%)` | :arrow_up: | | [request.go](https://codecov.io/gh/lni/dragonboat/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni#diff-cmVxdWVzdC5nbw==) | `93.31% <0.00%> (+0.32%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lni)

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[lni]

Thanks a lot for the fix.