CURLOPT_HEADERFUNCTION.3: match parameter name in synopsis and desc

Bug: #229
Reported-by: bsammon

docs/libcurl/opts/CURLOPT_HEADERFUNCTION.3

@@ -39,7 +39,7 @@ shown above.
 This function gets called by libcurl as soon as it has received header
 data. The header callback will be called once for each header and only
 complete header lines are passed on to the callback. Parsing headers is very
-easy using this. The size of the data pointed to by \fIptr\fP is \fIsize\fP
+easy using this. The size of the data pointed to by \fIbuffer\fP is \fIsize\fP
 multiplied with \fInmemb\fP. Do not assume that the header line is zero
 terminated! The pointer named \fIuserdata\fP is the one you set with the
 \fICURLOPT_HEADERDATA(3)\fP option. This callback function must return the