Complete refget → refget sequences naming update

[nsheff]

Summary

• Updates remaining instances where "refget" should be "refget sequences" in prose text
• Follows up on PR Update naming to refget sequences. See #94 #109 which did the initial naming update

Changes

• Line 27: "Refget was designed..." → "Refget sequences was designed..."
• Line 133: "Refget optionally allows..." → "Refget sequences optionally allows..."
• Line 137: "unique within the refget implementation" → "refget sequences implementation"
• Line 161: "by the refget implementation" → "refget sequences implementation"
• Line 392: "Refget server" → "Refget sequences server" (example)
• Line 479: "Refget implementations MUST support..." → "Refget sequences implementations..."

Not changed (intentionally)

• "refget Checksum Algorithm" (lines 108-109) — algorithm is shared across sequences and seqcol
• refget JSON field name (API spec)
• "Refget" as naming authority in appendix table
• Historical specification links

Fixes #94