[Fix] add missing "this" keyword

See @ben's comment on PR #1444[1].

[1]: #1444 (comment)

Author: Vipul Kumar

book/01-introduction/sections/help.asc

@@ -10,7 +10,7 @@ $ git <verb> --help
 $ man git-<verb>
 ----
 
-For example, you can get the manpage help for the `git config` command by running:(((git commands, help)))
+For example, you can get the manpage help for the `git config` command by running this:(((git commands, help)))
 
 [source,console]
 ----