Clang: fix syntax error in build.py

This was a copy-pasto from ceb49dda.

TBR=thakis

Bug: 1105518
Change-Id: I263cd1fbbe99b4c7435789099f41ad4d59b67805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300105
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788578}

tools/clang/scripts/build.py

@@ -414,7 +414,7 @@ def main():
     old_lib_dir = os.path.join(LLVM_BUILD_DIR, 'lib', 'clang', '11.0.0')
     if (os.path.isdir(old_lib_dir)):
       print('Removing old lib dir: ' + old_lib_dir)
-      RmTree(old_lib_dir
+      RmTree(old_lib_dir)
 
   if args.pgo and not args.bootstrap:
     print('--pgo requires --bootstrap')