Fix Hash.new(initial_capacity, &block) doc to use relevant example

[lachlan]

The Hash.new(initial_capacity = nil, &block : Hash(K, V), K -> V) comment's first code example calls a different constructor rather than the constructor being documented:

inventory = Hash(String, Int32).new(0) # => actually calls Hash.new(default_value : V, initial_capacity = nil)
inventory["socks"] = 3
inventory["pickles"] # => 0

Fixed to provide an example of how to use the Hash.new(initial_capacity = nil, &block : Hash(K, V), K -> V) constructor instead.