x/tools/gopls: CommentToMarkdown adds extra newlines when formatting code blocks

[hyangah]

gopls version 0.6.1
go version go1.16beta1 darwin/amd64

hover over make shows

Gopls trace:

[Trace - 20:54:39.323 PM] Received response 'textDocument/hover - (65)' in 0ms.
Result: {"contents":{"kind":"markdown","value":"```go\nfunc(t Type, size ...IntegerType) Type\n```\n\n[`make` on pkg.go.dev](https://pkg.go.dev/builtin?utm_source=gopls#make)\n\nThe make built\\-in function allocates and initializes an object of type\nslice, map, or chan \\(only\\)\\. Like new, the first argument is a type, not a\nvalue\\. Unlike new, make\\'s return type is the same as the type of its\nargument, not a pointer to it\\. The specification of the result depends on\nthe type\\:\n\n\n    Slice: The size specifies the length. The capacity of the slice is\n\n    equal to its length. A second integer argument may be provided to\n\n    specify a different capacity; it must be no smaller than the\n\n    length. For example, make([]int, 0, 10) allocates an underlying array\n\n    of size 10 and returns a slice of length 0 and capacity 10 that is\n\n    backed by this underlying array.\n\n    Map: An empty map is allocated with enough space to hold the\n\n    specified number of elements. The size may be omitted, in which case\n\n    a small starting size is allocated.\n\n    Channel: The channel's buffer is initialized with the specified\n\n    buffer capacity. If zero, or the size is omitted, the channel is\n\n    unbuffered.\n\n"},"range":{"start":{"line":31,"character":12},"end":{"line":31,"character":16}}}