Fix a typo in "gn help refs" output

An example in the help output shows the usage of "--as=buildfiles", but
the actual switch value is "buildfile".

Change-Id: I9e437fe38644eae7ba5dd5970bfa80581cd9e1e8
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16981
Reviewed-by: Brett Wilson <brettw@google.com>
Commit-Queue: Brett Wilson <brettw@google.com>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index e98a86a..4d912b6 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -1322,7 +1322,7 @@
   gn refs out/Debug //gn:gn
       Find all targets depending on the given exact target name.
 
-  gn refs out/Debug //base:i18n --as=buildfiles | xargs gvim
+  gn refs out/Debug //base:i18n --as=buildfile | xargs gvim
       Edit all .gn files containing references to //base:i18n
 
   gn refs out/Debug //base --all

src/gn/command_refs.cc

diff --git a/src/gn/command_refs.cc b/src/gn/command_refs.cc
index 9c83877..b751035 100644
--- a/src/gn/command_refs.cc
+++ b/src/gn/command_refs.cc
@@ -302,7 +302,7 @@
   gn refs out/Debug //gn:gn
       Find all targets depending on the given exact target name.
 
-  gn refs out/Debug //base:i18n --as=buildfiles | xargs gvim
+  gn refs out/Debug //base:i18n --as=buildfile | xargs gvim
       Edit all .gn files containing references to //base:i18n
 
   gn refs out/Debug //base --all