style: Fix RuboCop offence

```
lib/git_stats/git_data/comment_stat.rb:28:74: C: [Corrected] Style/RedundantStringEscape: Redundant escape of - inside string literal.
                  "{if ($0 ~ /^\\+#{escaped_string}/) adds++; if ($0 ~ /^\-#{escaped_string}/) dels++} " \
                                                                         ^^
```

lib/git_stats/git_data/comment_stat.rb

@@ -25,7 +25,7 @@ def calculate_stat
  escaped_string = escape_characters_in_string(commit.repo.comment_string)
  command = "git show #{commit.sha} | " \
  "awk 'BEGIN {adds=0; dels=0} " \
- "{if ($0 ~ /^\\+#{escaped_string}/) adds++; if ($0 ~ /^\-#{escaped_string}/) dels++} " \
+ "{if ($0 ~ /^\\+#{escaped_string}/) adds++; if ($0 ~ /^-#{escaped_string}/) dels++} " \
  "END {print adds \" insertions \" dels \" deletes\"}'"
  stat_line = commit.repo.run(command).lines.to_a[0]
  if stat_line.blank?