Treat Mainnet And Testnet As Common Nouns, Lowercasing When Appropriate

Suggested by @saivann (thanks!)

_includes/example_payment_processing.md

@@ -102,7 +102,7 @@ within the PaymentRequest.
 ## SSL Signature method
 request.pki_type = "x509+sha256"  ## Default: none
 
-## Mainnet or Testnet?
+## Mainnet or testnet?
 details.network = "test"  ## Default: main
 
 ## Postback URL

_includes/guide_p2p_network.md

@@ -50,8 +50,8 @@ provide dynamic DNS seed servers which automatically get IP addresses
 of active nodes by scanning the network; others provide static DNS
 seeds that are updated manually and are more likely to provide IP
 addresses for inactive nodes. In either case, nodes are added to the
-DNS seed if they run on the default Bitcoin ports of 8333 for Mainnet
-or 18333 for Testnet.
+DNS seed if they run on the default Bitcoin ports of 8333 for mainnet
+or 18333 for testnet.
 
 <!-- paragraph below based on Greg Maxwell's email in
      http://comments.gmane.org/gmane.comp.bitcoin.devel/5378 -->

_includes/guide_payment_processing.md

@@ -133,7 +133,7 @@ To specify an amount directly for copying and pasting, you must provide
 the address, the amount, and the denomination. An expiration time for
 the offer may also be specified.  For example:
 
-(Note: all examples in this section use Testnet addresses.)
+(Note: all examples in this section use testnet addresses.)
 
 {% endautocrossref %}
 

_includes/ref_core_rpc_intro.md

@@ -4,7 +4,7 @@
 
 Bitcoin Core RPCs accept and return hashes in the reverse of their
 normal byte order. For example, the Unix `sha256sum` command would display the
-SHA256(SHA256()) hash of Mainnet block 300,000's header as the
+SHA256(SHA256()) hash of mainnet block 300,000's header as the
 following string:
 
     5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc820000000000000000