Fix the introduction version for the new 2.1 functions

doc/src/manual/cowboy_req.cert.asciidoc

@@ -54,7 +54,7 @@ The client TLS certificate.
 
 == Changelog
 
-* *2.0*: Function introduced.
+* *2.1*: Function introduced.
 
 == Examples
 

doc/src/manual/cowboy_req.inform.asciidoc

@@ -56,7 +56,7 @@ The atom `ok` is always returned. It can be safely ignored.
 
 == Changelog
 
-* *2.0*: Function introduced.
+* *2.1*: Function introduced.
 
 == Examples
 

doc/src/manual/cowboy_req.sock.asciidoc

@@ -34,7 +34,7 @@ The socket's local IP address and port number.
 
 == Changelog
 
-* *2.0*: Function introduced.
+* *2.1*: Function introduced.
 
 == Examples