Fix docs for Indexable::Mutable#map!

[HertzDevil]

#11267 caused TODO: add as constant to overwrite the original documentation. This fixes it.

[straight-shoota]

Oh that's weird doc association.
Should probably be fixed with #11268, though

[HertzDevil]

I don't think so. The following:

# Invokes the given block for each element of `self`, replacing the element
# with the value returned by the block. Returns `self`.
#
# ```
# a = [1, 2, 3]
# a.map! { |x| x * x }
# a # => [1, 4, 9]
# ```
{% begin %}
{% if compare_versions(Crystal::VERSION, "1.1.1") >= 0 %} # TODO: add as constant
def map!(& : T -> _) : self
{% else %}
def map!(&) # it doesn't compile with the type annotation in the 1.0.0 compiler
{% end %}

expands to:

# Invokes the given block for each element of `self`, replacing the element
# with the value returned by the block. Returns `self`.
#
# ```
# a = [1, 2, 3]
# a.map! { |x| x * x }
# a # => [1, 4, 9]
# ```

 # TODO: add as constant
def map!(& : T -> _) : self

Likewise, on versions before 1.1.1 the original documentation will still be completely disconnected from the method. So this probably has nothing to do with trailing comments on the def's line. I also verified that # TODO: add as constant does not appear in the generated docs after this PR.