Minor comment corrections in cpu_sa.cpp

[mcfarljm]

• Correct sign in description of flip acceptance
• Clarify the source of -2 * var in delta energy calculation. The previous description was not strictly correct because it did not include the h portion in description of delta energy. I also found that it did not really explain the source of -2 * var.

[codecov-io]

Codecov Report

Merging #88 (c51d0e3) into master (b39f760) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
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  Coverage   98.57%   98.57%           
=======================================
  Files           2        2           
  Lines          70       70           
=======================================
  Hits           69       69           
  Misses          1        1           

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