Doc: change Precise to Trusty in gitian-building.md

2cecb24600 changed "precise" to "trusty" in the shell command, but didn't change the text above it accordingly.  This commit fixes that.

doc/gitian-building.md

@@ -313,7 +313,7 @@ Setting up the Gitian image
 -------------------------
 
 Gitian needs a virtual image of the operating system to build in.
-Currently this is Ubuntu Precise x86_64.
+Currently this is Ubuntu Trusty x86_64.
 This image will be copied and used every time that a build is started to
 make sure that the build is deterministic.
 Creating the image will take a while, but only has to be done once.