runtime(doc): Tweak documentation style a bit

closes: #16164

Signed-off-by: h-east <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index d0f0c7b..ac10590 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Nov 26
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Dec 03
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -3789,7 +3789,7 @@
 
 getcellpixels()						*getcellpixels()*
 		Returns a |List| of terminal cell pixel size.
-		List format is [xpixels, ypixels].
+		List format is [xpixel, ypixel].
 
 		Only works on Unix (terminal and gVim) and Windows (gVim only).
 		Returns [] on other systems or on failure.