Fix typo in Linux build instructions

Change-Id: Ibd3291ac69042141ad9809021abcb6d7a18b27cf
Reviewed-on: https://chromium-review.googlesource.com/779186Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517814}

docs/linux_build_instructions.md

@@ -176,7 +176,7 @@ See [related bug](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=808181).
 #### ccache
 
 You can use [ccache](https://ccache.samba.org) to speed up local builds (again,
-this is not useful if you're using a Googler using Goma).
+this is not useful if you're a Googler using Goma).
 
 Increase your ccache hit rate by setting `CCACHE_BASEDIR` to a parent directory
 that the working directories all have in common (e.g.,