Renumber Float16 to 0277, oops...

Author: airspeedswift

proposals/0276-float16.md renamed to proposals/0277-float16.md

@@ -1,6 +1,6 @@
 # Float16
 
-* Proposal: [SE-0276](0276-float16.md)
+* Proposal: [SE-0277](0277-float16.md)
 * Author: [Stephen Canon](https://github.com/stephentyrone)
 * Review Manager: [Ben Cohen](https://github.com/airspeedswift)
 * Pitch thread: [Add Float16](https://forums.swift.org/t/add-float16/33019)