Add missing space to build_timeout description

Change-Id: I2b4ceb5c4c795903454873a6d4882c563499186a
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index dfcbaba..8499cac 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -1100,7 +1100,7 @@
 # value)
 #build_interval=1
 
-# Timeout in seconds to wait for a volume to becomeavailable.
+# Timeout in seconds to wait for a volume to become available.
 # (integer value)
 #build_timeout=300