doc: be consistent in Deprecated sentence case

[bradfitz]

$ git grep "// Deprecated: "
archive/zip/struct.go:  CompressedSize     uint32 // Deprecated: Use CompressedSize64 instead.
archive/zip/struct.go:  UncompressedSize   uint32 // Deprecated: Use UncompressedSize64 instead.
cmd/compile/internal/gc/type.go:// Deprecated: New code should use Slice instead.
cmd/internal/obj/textflag.go:   // Deprecated: Not implemented, do not use.
compress/flate/inflate.go:// Deprecated: No longer returned.
compress/flate/inflate.go:// Deprecated: No longer returned.
go/doc/doc.go:  // Deprecated: For backward compatibility Bugs is still populated,
image/jpeg/reader.go:// Deprecated: Reader is deprecated.
net/http/httputil/persist.go:// Deprecated: use the Server in package net/http instead.
net/http/httputil/persist.go:// Deprecated: use the Server in package net/http instead.
net/http/httputil/persist.go:// Deprecated: use Client or Transport in package net/http instead.
net/http/httputil/persist.go:// Deprecated: use the Client or Transport in package net/http instead.
net/http/httputil/persist.go:// Deprecated: use the Client or Transport in package net/http instead.
net/http/request.go:    // Deprecated: use the Context and WithContext methods
net/http/transport.go:// Deprecated: Use Request.Cancel instead. CancelRequest cannot cancel
syscall/exec_plan9.go:// Deprecated: Use SlicePtrFromStrings instead.
syscall/exec_unix.go:// Deprecated: Use SlicePtrFromStrings instead.
syscall/syscall.go:// Deprecated: Use ByteSliceFromString instead.
syscall/syscall.go:// Deprecated: Use BytePtrFromString instead.
syscall/syscall_windows.go:// Deprecated: Use UTF16FromString instead.
syscall/syscall_windows.go:// Deprecated: Use UTF16PtrFromString instead.
syscall/syscall_windows.go:// Deprecated: Use FormatMessage from golang.org/x/sys/windows instead.

I guess the rule is to start with a capital after Deprecated.

[gopherbot]

CL https://golang.org/cl/21544 mentions this issue.