docs/btf: fix the missing section marks

The section titles of 3.4 and 3.5 are not marked correctly.

Signed-off-by: Gary Lin <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

Documentation/bpf/btf.rst

···
     #define BPF_LINE_INFO_LINE_COL(line_col)        ((line_col) & 0x3ff)
 
 3.4 BPF_{PROG,MAP}_GET_NEXT_ID
+==============================
 
 In kernel, every loaded program, map or btf has a unique id. The id won't
 change during the lifetime of a program, map, or btf.
···
 inspection tool can inspect all programs and maps.
 
 3.5 BPF_{PROG,MAP}_GET_FD_BY_ID
+===============================
 
 An introspection tool cannot use id to get details about program or maps.
 A file descriptor needs to be obtained first for reference-counting purpose.