clean up non-ASCII comments

Change-Id: I07ad00133f6a938de70a94024a0ebe36c6c542bb
Reviewed-on: https://skia-review.googlesource.com/6524
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index f1c08d7..c9456db 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -328,7 +328,7 @@
     //  -------------------------  < FLAGS_overheadGoal
     //  overhead + N * Bench Time
     //
-    // where bench_plus_overhead ≈ overhead + Bench Time.
+    // where bench_plus_overhead ~=~ overhead + Bench Time.
     //
     // Doing some math, we get:
     //