HADOOP-16373. Fix typo in FileSystemShell#test documentation (#968)

hadoop-common-project/hadoop-common/src/site/markdown/FileSystemShell.md

@@ -737,7 +737,7 @@ Usage: `hadoop fs -test -[defsz] URI`
 
 Options:
 
-* -d: f the path is a directory, return 0.
+* -d: if the path is a directory, return 0.
 * -e: if the path exists, return 0.
 * -f: if the path is a file, return 0.
 * -s: if the path is not empty, return 0.