man: fix example syntax with empty replace key and cs{diff,grep}

Author: lzaoral

doc/csdiff.h2m

@@ -3,7 +3,7 @@ csdiff - take two lists of defects and output either added or fixed ones
 
 [OPTIONS]
 The \fB\-\-filter\-file\fR option takes a list of JSON files in the following
-format.  Missing replace entry is equal to replace : "".
+format.  Missing replace entry is equal to "replace" : "".
 
 .RS 4
 .nf

doc/csgrep.h2m

@@ -3,7 +3,7 @@ csgrep - filter the list of defects by the specified regex-based predicates
 
 [OPTIONS]
 The \fB\-\-filter\-file\fR option takes a list of JSON files in the following
-format.  Missing replace entry is equal to replace : "".
+format.  Missing replace entry is equal to "replace" : "".
 
 .RS 4
 .nf