Fix a typo in the gn gen help for QtCreator generator (introduced in https://codereview.chromium.org/1883093002/)

BUG=

Review-Url: https://codereview.chromium.org/1979813002
Cr-Original-Commit-Position: refs/heads/master@{#393795}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 457e2cd70b7aeab763f7ee35bc60c62d36121c26
diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index f872304..7bdb6d8 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -293,7 +293,7 @@
     "  --root-target=<target_name>\n"
     "      Name of the root target for which the QtCreator project will be\n"
     "      generated to contain files of it and its dependencies. If unset, \n"
-    "      the whole build graph will be omitted.\n"
+    "      the whole build graph will be emitted.\n"
     "\n"
     "\n"
     "Eclipse IDE Support\n"