Fix wording on Mojo security docs.

TBR=dcheng

Bug: 789374
Change-Id: I68d41425e0cf376d1346546268fac60bd9a6dc3c
Reviewed-on: https://chromium-review.googlesource.com/807184Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521452}

docs/security/mojo.md

@@ -563,7 +563,7 @@ process, and not as a (potentially compromised) web renderer or other
 low-privilege process.
 
 * Use the standard pattern for instantiating `MojoWebUIController`. WebUI
-methods must only be exposed through a `MojoWebUIController` subclass.
+Mojo interfaces must only be exposed through a `MojoWebUIController` subclass.
 * If there is external functionality that the WebUI needs, make sure to route
 it through the Mojo interfaces implemented by the `MojoWebUIController`, to
 avoid circumventing access checks.