Fix docstring for Val(c) in order that c accepts a Symbol

[kimikage]

This was mentioned on the slack. Even though a Symbol instance is not an isbits value, Val{:symbol} is valid.

Strictly speaking, a value of the composite type consisting of symbols and isbits values ​​is also available. However, I prefer simplicity here to strictness. I don't know whether there are any exceptions other than the Symbol.

julia> isbits(:symbol)
false

julia> Val(:symbol)
Val{:symbol}()

julia> isbits((:foo, 1))
false

julia> Val((:foo, 1))
Val{(:foo, 1)}()