[DOC] Use unpack1 instead of unpack(template)[0] [ci skip]

ext/socket/basicsocket.c

@@ -306,7 +306,7 @@ bsock_setsockopt(int argc, VALUE *argv, VALUE sock)
  *   ipttl = sock.getsockopt(:IP, :TTL).int
  *
  *   optval = sock.getsockopt(Socket::IPPROTO_IP, Socket::IP_TTL)
- *   ipttl = optval.unpack("i")[0]
+ *   ipttl = optval.unpack1("i")
  *
  * Option values may be structs. Decoding them can be complex as it involves
  * examining your system headers to determine the correct definition. An

io.c

@@ -3324,7 +3324,7 @@ io_write_nonblock(rb_execution_context_t *ec, VALUE io, VALUE str, VALUE ex)
  *     # each record is prefixed by its 32-bit length
  *     open("variable-record-file") do |f|
  *       while len = f.read(4)
- *         len = len.unpack("N")[0]   # 32-bit length
+ *         len = len.unpack1("N")     # 32-bit length
  *         record = f.read(len)       # This returns a string even if len is 0.
  *       end
  *     end

string.c

@@ -10831,7 +10831,7 @@ enc_str_scrub(rb_encoding *enc, VALUE str, VALUE repl, int cr)
  *
  *     "abc\u3042\x81".scrub #=> "abc\u3042\uFFFD"
  *     "abc\u3042\x81".scrub("*") #=> "abc\u3042*"
- *     "abc\u3042\xE3\x80".scrub{|bytes| '<'+bytes.unpack('H*')[0]+'>' } #=> "abc\u3042<e380>"
+ *     "abc\u3042\xE3\x80".scrub{|bytes| '<'+bytes.unpack1('H*')+'>' } #=> "abc\u3042<e380>"
  */
 static VALUE
 str_scrub(int argc, VALUE *argv, VALUE str)
@@ -10853,7 +10853,7 @@ str_scrub(int argc, VALUE *argv, VALUE str)
  *
  *     "abc\u3042\x81".scrub! #=> "abc\u3042\uFFFD"
  *     "abc\u3042\x81".scrub!("*") #=> "abc\u3042*"
- *     "abc\u3042\xE3\x80".scrub!{|bytes| '<'+bytes.unpack('H*')[0]+'>' } #=> "abc\u3042<e380>"
+ *     "abc\u3042\xE3\x80".scrub!{|bytes| '<'+bytes.unpack1('H*')+'>' } #=> "abc\u3042<e380>"
  */
 static VALUE
 str_scrub_bang(int argc, VALUE *argv, VALUE str)