Fix format string crash.

[hhanesand]

Was seeing a BAD_ACCESS crash on one of my log statements because I was passing a string with a %@, and the NSAssert function expects a format string, not a literal.

[OSSPolice]

	2 Warnings
⚠️	Any changes to library code should be reflected in the CHANGELOG. Please consider adding a note there about your change.
⚠️	The library files were changed, but the tests remained unmodified. Consider updating or adding to the tests to match the library changes.

Generated by 🚫 Danger

[codecov-io]

Codecov Report

Merging #1066 into master will increase coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1066      +/-   ##
=========================================
+ Coverage   37.52%   38.1%   +0.58%     
=========================================
  Files          27      27              
  Lines        3944    3944              
=========================================
+ Hits         1480    1503      +23     
+ Misses       2464    2441      -23

Impacted Files	Coverage Δ
Classes/DDFileLogger.m	57.16% <0%> (+0.26%)	⬆️
Classes/DDOSLogger.m	61.53% <0%> (+40.38%)	⬆️

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[sushichop]

LGTM👍