log doc: the argument to --encoding is not optional

 $ git log --encoding
 fatal: Option '--encoding' requires a value
 $ git rev-list --encoding
 fatal: Option '--encoding' requires a value

The argument to --encoding has always been mandatory.  Unfortunately
manpages like git-rev-list(1), git-log(1), and git-show(1) have
described the option's syntax as "--encoding[=<encoding>]" since it
was first documented.  Clarify by removing the extra brackets.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rev-list.txt

@@ -40,7 +40,7 @@ SYNOPSIS
 	     [ \--right-only ]
 	     [ \--cherry-mark ]
 	     [ \--cherry-pick ]
-	     [ \--encoding[=<encoding>] ]
+	     [ \--encoding=<encoding> ]
 	     [ \--(author|committer|grep)=<pattern> ]
 	     [ \--regexp-ignore-case | -i ]
 	     [ \--extended-regexp | -E ]

Documentation/pretty-options.txt

@@ -28,7 +28,7 @@ people using 80-column terminals.
 	This is a shorthand for "--pretty=oneline --abbrev-commit"
 	used together.
 
---encoding[=<encoding>]::
+--encoding=<encoding>::
 	The commit objects record the encoding used for the log message
 	in their encoding header; this option can be used to tell the
 	command to re-code the commit log message in the encoding