Remove copied env variable

Author: KyleAMathews

benchmarks/image-processing/README.md

@@ -3,7 +3,7 @@
 Benchmarks Gatsby's image processing capabilities
 by downloading and processing images.
 
-Defaults to building a site with 100 image pages. Set the `NUM_IMAGES` environment variable to change that e.g. `NUM_IMAGES=1000 MAX_NUM_ROWS=100 gatsby build`
+Defaults to building a site with 100 image pages. Set the `NUM_IMAGES` environment variable to change that e.g. `NUM_IMAGES=1000 gatsby build`
 
 The max number of images you can process is 1300.