Fix some formatting issues in the manual

doc/src/manual/cowboy_req.inform.asciidoc

@@ -43,7 +43,7 @@ The status code for the response.
 Headers::
 
 The response headers.
-
++
 Header names must be given as lowercase binary strings.
 
 Req::

doc/src/manual/cowboy_req.push.asciidoc

@@ -57,7 +57,7 @@ The status code for the response.
 Headers::
 
 The response headers.
-
++
 Header names must be given as lowercase binary strings.
 
 Req::

doc/src/manual/cowboy_req.reply.asciidoc

@@ -57,7 +57,7 @@ The status code for the response.
 Headers::
 
 The response headers.
-
++
 Header names must be given as lowercase binary strings.
 
 Body::

doc/src/manual/cowboy_req.stream_reply.asciidoc

@@ -61,7 +61,7 @@ The status code for the response.
 Headers::
 
 The response headers.
-
++
 Header names must be given as lowercase binary strings.
 
 Req::