docs: fix broken links in i18n modue

We want to talk about the kwargs to the custom_target() function, but
tried to link to custom_tgt instead, which is not a function. It is an
object, but this was the wrong reference method for a type.

We want the function anyway, so use that.

docs/markdown/i18n-module.md

@@ -46,7 +46,7 @@ This function also defines targets for maintainers to use:
 ### i18n.merge_file()
 
 This merges translations into a text file using `msgfmt`. See
-[[custom_tgt]]
+[[custom_target]]
 for normal keywords. In addition it accepts these keywords:
 
 * `output`: same as `custom_target` but only accepts one item
@@ -63,7 +63,7 @@ for normal keywords. In addition it accepts these keywords:
 ### i18n.itstool_join()
 
 This joins translations into a XML file using `itstool`. See
-[[custom_tgt]]
+[[custom_target]]
 for normal keywords. In addition it accepts these keywords:
 
 * `output`: same as `custom_target` but only accepts one item