Fix typo.

`SwiftSnytax` -> `SwiftSyntax`

Author: dan-zheng

update-toolchain.py

@@ -46,7 +46,7 @@ def main():
     parser = argparse.ArgumentParser(
         formatter_class=argparse.RawDescriptionHelpFormatter,
         description='''
-Build and re-install SwiftSytnax for an existing toolchain.
+Build and re-install SwiftSyntax for an existing toolchain.
 
 This script calls ./build-script internally to rebuild SwiftSyntax using an
 existing toolchain and install the build artifacts back to the toolchain.
@@ -75,4 +75,4 @@ def main():
         module_dir=module_dir)
 
 if __name__ == '__main__':
-    main()
+    main()