release-set-version.py: fixed invalid escape sequence

```
SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
```

Author: firewave

tools/release-set-version.py

@@ -81,7 +81,7 @@ def check_sed(args):
     check_sed(['-i', '-r', f's/<[?]define ProductName[ ]*=.*/<?define ProductName = "Cppcheck $(var.Platform) {cppcheck_version_string}" ?>/', 'win_installer/productInfo.wxi'])
     check_sed(['-i', '-r', f's/<[?]define ProductVersion[ ]*=.*/<?define ProductVersion = "{new_version}" ?>/', 'win_installer/productInfo.wxi'])
     for g in glob.glob('man/*.md'):
-        check_sed(['-i', '-r', f's/subtitle: Version 2\.[0-9].*/subtitle: Version {cppcheck_version_string}/', g])
+        check_sed(['-i', '-r', f's/subtitle: Version 2\\.[0-9].*/subtitle: Version {cppcheck_version_string}/', g])
     print('Versions have been changed.')
     print('')
     print('Please double check these results below:')