spelling: add userbase

Not in an official dictionary, but per [1] still well defined and
known in that context.

[1]: https://en.wiktionary.org/wiki/userbase

Signed-off-by: Christian Ehrhardt <[email protected]>

Author: cpaelzer

docs/.custom_wordlist.txt

@@ -245,6 +245,7 @@ Unshareable
 unshareable
 uploader
 uploaders
+userbase
 utils
 UVF
 vendored