docs: update tense in Changelog

Signed-off-by: Yukai Chou <[email protected]>

Author: muzimuzhi

doc/generic/pgf/CHANGELOG.md

@@ -22,7 +22,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Treat varargs for `min()` and `max()` in `luamath` pgf library pgf-tikz/pgfplots#492 #1359
 - Fixed support for the `\tikz` command in the `dvisvgm4ht` driver for TeX4ht
 - Wrong key name in unknown decoration option error #1082
-- `gnuplot` is always run when the function contains leading or trailing spaces #1397
+- `gnuplot` was always run when the function contained leading or trailing spaces #1397
 
 ### Changed