Merge pull request #1790 from arjclark/fix.doc_typo_in_var_name

Fix typo in docs

doc/cug.tex

@@ -2071,7 +2071,7 @@ \subsection{Task Retry On Failure}
 \vspace{3mm}
 
 Tasks can be configured to retry a number of times if they fail.
-An environment variable \lstinline=$CYLC_TASK_TRY_NUBMER= increments
+An environment variable \lstinline=$CYLC_TASK_TRY_NUMBER= increments
 from $1$ on each successive try, and is passed to the task to allow
 different behaviour on the retry:
 \lstset{language=suiterc}