doc: fix for plurality typo on bgwriter doc sentence

Reported-by: Justin Pryzby

Discussion: https://postgr.es/m/20191106022353.GX4999@telsasoft.com

Backpatch-through: 11, 12

doc/src/sgml/bgworker.sgml

@@ -23,7 +23,7 @@
    There are considerable robustness and security risks in using background
    worker processes because, being written in the <literal>C</literal> language,
    they have unrestricted access to data.  Administrators wishing to enable
-   modules that include background worker processs should exercise extreme
+   modules that include background worker processes should exercise extreme
    caution.  Only carefully audited modules should be permitted to run
    background worker processes.
   </para>