Replace "multi" with "fat" in build documentation.

ios/build/tools/setup-gn.config

 [goma]
 # Controls whether goma is enabled or not. If you generally use goma but
 # want to disable goma for a single build, consider using the environment
 # variable GOMA_DISABLED.
 enabled = False
 install = "$GOMA_DIR"
 
 [xcode]
 # Controls settings for the generated Xcode project. If jobs is non-zero
 # it will be passed to the ninja invocation in Xcode project.
 jobs = 0
 
 [build]
 # Controls the build output. The only supported values are "64-bit", "32-bit"
-# and "multi" (for a fat binary supporting both "32-bit" and "64-bit" cpus).
+# and "fat" (for a fat binary supporting both "32-bit" and "64-bit" cpus).
 arch = "64-bit"
 
 [gn_args]
 # Values in that section will be copied verbatim in the generated args.gn file.
 target_os = "ios"
 
 [filters]
 # List of target files to pass to --filters argument of gn gen when generating
 # the Xcode project. By default, list all targets from ios/ and ios_internal/
 # and the targets corresponding to the unit tests run on the bots.
 filter_0 = "//base:base_unittests"
 filter_1 = "//components:components_unittests"
 filter_2 = "//crypto:crypto_unittests"
 filter_3 = "//google_apis:google_apis_unittests"
 filter_4 = "//ios/*"
 filter_5 = "//ios_internal/*"
 filter_6 = "//net:net_unittests"
 filter_7 = "//skia:skia_unittests"
 filter_8 = "//sql:sql_unittests"
 filter_9 = "//ui/base:ui_base_unittests"
 filter_a = "//ui/gfx:gfx_unittests"
 filter_b = "//url:url_unittests"