doc: typo fix from Harold via openresty#446.

Author: agentzh

README.markdown

@@ -3887,7 +3887,7 @@ Set the current request's request header named `header_name` to value `header_va
 
 By default, all the subrequests subsequently initiated by [ngx.location.capture](#ngxlocationcapture) and [ngx.location.capture_multi](#ngxlocationcapture_multi) will inherit the new header.
 
-Here is an example of setting the `Content-Length` header:
+Here is an example of setting the `Content-Type` header:
 
 ```lua
 

doc/HttpLuaModule.wiki

@@ -3221,7 +3221,7 @@ Set the current request's request header named <code>header_name</code> to value
 
 By default, all the subrequests subsequently initiated by [[#ngx.location.capture|ngx.location.capture]] and [[#ngx.location.capture_multi|ngx.location.capture_multi]] will inherit the new header.
 
-Here is an example of setting the <code>Content-Length</code> header:
+Here is an example of setting the <code>Content-Type</code> header:
 
 <geshi lang="lua">
     ngx.req.set_header("Content-Type", "text/css")