docs: fix Blocks.Sum docstring

[AayushSabharwal]

MWE:

julia> Sum(;name=:a, input.nin=2)
ERROR: syntax: invalid keyword argument name "input.nin" around REPL[8]:1
Stacktrace:
 [1] top-level scope
   @ REPL[8]:

julia> @macroexpand @mtkmodel Sum begin
           @components begin
               input = RealInput(; nin)
               output = RealOutput()
           end
           @equations begin
               output.u ~ sum(input.u)
           end
       end
:(Sum = (ModelingToolkit.Model)(((; name, input__nin = nothing)->begin
                  #= /Users/aayush/.julia/packages/ModelingToolkit/xLMQM/src/systems/model_parsing.jl:242 =#
                  begin
                      begin
                          input = RealInput(name = :input; nin = input__nin)
                          output = RealOutput(name = :output)
                      end
                      (ODESystem)((Equation)[output.u ~ sum(input.u)], t, [], []; systems = [input, output], name, gui_metadata = nothing)
                  end
              end), Dict{Symbol, Any}(:components => [[:input, :RealInput], [:output, :RealOutput]], :kwargs => Dict{Symbol, Any}(), :independent_variable => t, :equations => ["output.u ~ sum(input.u)"]), false))

julia> Sum(;name=:a, input__nin=2)
Model a with 1 equations
...

[codecov]

Codecov Report

Merging #201 (68bd726) into main (a5c2c01) will decrease coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
- Coverage   56.08%   55.76%   -0.32%     
==========================================
  Files          48       48              
  Lines        1578     1587       +9     
==========================================
  Hits          885      885              
- Misses        693      702       +9     

Impacted Files	Coverage Δ
src/Blocks/math.jl	95.00% <ø> (ø)

... and 2 files with indirect coverage changes

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