Doc: replace "user name" with "username"

Change-Id: I83e634b9efd407f0dd36f1b0f921a46cd33cd591
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

doc/src/howto/creator-tips.qdoc

@@ -234,7 +234,7 @@
         \li \c %MONTH%: Month
         \li \c %DAY%: Day of the month
         \li \c %DATE%: Date
-        \li \c %USER%: User name
+        \li \c %USER%: Username
         \li \c %FILENAME%: File name
         \li \c %CLASS%: Class name (if applicable)
         \li \c %$VARIABLE%: Contents of environment variable \c{VARIABLE}.