Fix a typo in Clang tool refactoring docs

runtime => run time

Change-Id: I9fcc4b0c8b6156676985428b9eec7a6f3cb3fd34
Reviewed-on: https://chromium-review.googlesource.com/508120
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#472803}

docs/clang_tool_refactoring.md

@@ -87,7 +87,7 @@ While clang has a [`clang::tooling::RefactoringTool`](http://clang.llvm.org/doxy
 to automatically apply the generated replacements and save the results, it
 doesn't work well for Chromium:
 
-*   Clang tools run actions serially, so runtime scales poorly to tens of
+*   Clang tools run actions serially, so run time scales poorly to tens of
     thousands of files.
 *   A parsing error in any file (quite common in NaCl source) prevents any of
     the generated replacements from being applied.