[gce] Specify fx command for gce subcommands in README.md examples

All fx subcommands need to be prefixed with 'fx'.

Change-Id: Ie051bb092678f9a9b468328c73b3d756cdb85158

gce/README.md

diff --git a/gce/README.md b/gce/README.md
index 5dd42d4..011e2cf 100644
--- a/gce/README.md
+++ b/gce/README.md
@@ -27,10 +27,10 @@
 cd $FUCHSIA_ROOT
 fx set x64 --release
 fx full-build
-gce create-fuchsia-image
-gce create-instance
+fx gce create-fuchsia-image
+fx gce create-instance
 sleep 60
-gce serial
+fx gce serial
 ```
 
 ## How the sausage is made