net: nfc: Format userspace interface subsection headings

Subsection headings of "Userspace interface" is written in normal
paragraph, all-capped. Properly format them as reST section headings.

Signed-off-by: Bagas Sanjaya <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

Documentation/networking/nfc.rst

···
 The userspace interface is divided in control operations and low-level data
 exchange operation.
 
-CONTROL OPERATIONS:
+Control operations
+------------------
 
 Generic netlink is used to implement the interface to the control operations.
 The operations are composed by commands and events, all listed below:
···
 All polling operations requested through one netlink socket are stopped when
 it's closed.
 
-LOW-LEVEL DATA EXCHANGE:
+Low-level data exchange
+-----------------------
 
 The userspace must use PF_NFC sockets to perform any data communication with
 targets. All NFC sockets use AF_NFC::