Update generated manual

MANUAL.txt

@@ -168,6 +168,12 @@ Options:
           Use the much slower but stronger Zopfli compressor for main compression trials.
           Recommended use is with '-o max' and '--fast'.
 
+      --zi <iterations>
+          Set the number of iterations to use for Zopfli compression. Using fewer iterations may
+          speed up compression for large files. This option requires '--zopfli' to be set.
+          
+          [default: 15]
+
       --timeout <secs>
           Maximum amount of time, in seconds, to spend on optimizations. Oxipng will check the
           timeout before each transformation or compression trial, and will stop trying to optimize