fix spelling correctly

James Zern pointed out that the misspelled word "ot" should be changed
to "of", not "to".

apps/avifgainmaputil/swapbase_command.cc

diff --git a/apps/avifgainmaputil/swapbase_command.cc b/apps/avifgainmaputil/swapbase_command.cc
index f714108..fbd74c2 100644
--- a/apps/avifgainmaputil/swapbase_command.cc
+++ b/apps/avifgainmaputil/swapbase_command.cc
@@ -116,7 +116,7 @@
           "swapbase",
           "Swaps the base and alternate images (e.g. if the base image is SDR "
           "and the alternate is HDR, makes the base HDR). The alternate image "
-          "is the result to fully applying the gain map.") {
+          "is the result of fully applying the gain map.") {
   argparse_.add_argument(arg_input_filename_, "input_filename");
   argparse_.add_argument(arg_output_filename_, "output_filename");
   arg_image_read_.Init(argparse_);