doc: correct the 50 characters soft limit

The soft limit of the first line of the commit message should be
"no more than 50 characters" or "50 characters or less", but not
"less than 50 character".

Signed-off-by: 谢致邦 (XIE Zhibang) <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Red54

Documentation/git-commit.txt

@@ -541,7 +541,7 @@ DISCUSSION
 ----------
 
 Though not required, it's a good idea to begin the commit message
-with a single short (less than 50 character) line summarizing the
+with a single short (no more than 50 characters) line summarizing the
 change, followed by a blank line and then a more thorough description.
 The text up to the first blank line in a commit message is treated
 as the commit title, and that title is used throughout Git.