Fix typo in symbolization.

When redesigning stripping of path prefixes on the app binary name, I
managed to miss a ":", resulting in all binary names under "/system/"
being truncated to a single character...

Bug: 
Change-Id: I7e58d5b6593e5cb37b065c9fbc0d63448f29fb0e
Reviewed-on: https://chromium-review.googlesource.com/624814
Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496068}

build/fuchsia/runner_common.py

@@ -275,7 +275,7 @@ def _FindDebugBinary(entry, file_mapping):
   if binary.startswith(cwd_prefix):
     binary = binary[len(cwd_prefix):]
   elif binary.startswith(system_prefix):
-    binary = binary[len(system_prefix)]
+    binary = binary[len(system_prefix):]
   # Allow any other paths to pass-through; sometimes neither prefix is present.
 
   if binary in file_mapping: