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 <glin@suse.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/btf.rst

@@ -578,6 +578,7 @@ For line_info, the line number and column number are defined as below:
     #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.
@@ -587,6 +588,7 @@ each command, to user space, for bpf program or maps, respectively, so an
 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.