doc: fix grammar error in process.md

PR-URL: #19641
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/process.md

@@ -207,7 +207,7 @@ changes:
     description: Not handling Promise rejections has been deprecated.
   - version: v6.6.0
     pr-url: https://github.com/nodejs/node/pull/8223
-    description: Unhandled Promise rejections have been will now emit
+    description: Unhandled Promise rejections will now emit
                  a process warning.
 -->