Add PR number of NEWS bit for theme improvements

Author: jiho

NEWS

@@ -1,7 +1,7 @@
 ggplot2 0.9.3.1.99
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-* Marginal improvements to `theme_bw` and `theme_classic`
+* Marginal improvements to `theme_bw` and `theme_classic` (@jiho, #934)
 
 * `stat_ellipse()` adds data ellipses. It supports bivariate normal and t distributions,
   as well as a euclidian distance circle. (@jofrhwld, #926)