[tools] Fix typo in build.py

Closes https://github.com/dart-lang/sdk/pull/44580
https://github.com/dart-lang/sdk/pull/44580

GitOrigin-RevId: 43ebbcec6ee358d04f19bdd2850b2aa8f9718ad5
Change-Id: Ia10816097d43f9ee1062d846326a680767033165
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177520
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
diff --git a/tools/build.py b/tools/build.py
index 9062ef5..513bef8 100755
--- a/tools/build.py
+++ b/tools/build.py
@@ -29,7 +29,7 @@
 
 def BuildOptions():
     parser = argparse.ArgumentParser(
-        description='Runs GN (if ncecessary) followed by ninja',
+        description='Runs GN (if necessary) followed by ninja',
         formatter_class=argparse.ArgumentDefaultsHelpFormatter)
 
     config_group = parser.add_argument_group('Configuration Related Arguments')